Inter-cylinder air-fuel ratio imbalance determination apparatus for internal combustion engine

ABSTRACT

An inter-cylinder air-fuel ratio imbalance determination apparatus (determination apparatus) according to the present invention obtains, based on the output value of the air-fuel ratio sensor, an imbalance determination parameter which becomes larger as an air-fuel ratio fluctuation of an exhaust gas passing through a position at which the air-fuel ratio sensor is disposed becomes larger, during a parameter obtaining period. The determination apparatus energizes the heater of the air-fuel ratio sensor in such a manner that a temperature of the air-fuel ratio element during the parameter obtaining period is higher than a temperature of the air-fuel ratio element during a period other than the parameter obtaining period. Accordingly, the imbalance determination parameter is obtained while the responsiveness of the air-fuel ratio sensor is high, and thus, the inter-cylinder air-fuel-ratio imbalance determination having a high accuracy can be made.

TECHNICAL FIELD

The present invention relates to an “inter-cylinder air-fuel ratioimbalance determination apparatus for an internal combustion engine,”which is applied to a multi-cylinder internal combustion engine, andwhich can determine (monitor/detect) that a degree of imbalance amongthe air-fuel ratios of air-fuel mixtures, each supplied to each ofcylinders (inter-cylinder air-fuel ratio imbalance; inter-cylinderair-fuel ratio variation; or inter-cylinder air-fuel rationon-uniformity) has increased excessively.

BACKGROUND ART

Conventionally, as shown in FIG. 1, there has been widely known anair-fuel ratio control apparatus which includes a three-way catalyst(53) disposed in an exhaust passage of an internal combustion engine,and an upstream air-fuel ratio sensor (67) and a downstream air-fuelratio sensor (68) that are disposed upstream and downstream,respectively, of the three-way catalyst (53) in the exhaust passage.

This air-fuel ratio control apparatus calculates, based on the outputsof the upstream and downstream air-fuel ratio sensors, an “air-fuelratio feedback amount for having the air-fuel ratio of the air-fuelmixture supplied to the engine (air-fuel ratio of the engine) coincidewith the stoichiometric air-fuel ratio such that the air-fuel ratio ofthe engine coincides with the stoichiometric air-fuel ratio, and isconfigured so as to feedback-control the air-fuel ratio of the enginebased on the air-fuel ratio feedback amount. Furthermore, there has beenalso widely known an air-fuel ratio control apparatus which calculates,based on the output of the upstream air-fuel ratio sensor only, an“air-fuel ratio feedback amount for having the air-fuel ratio of theengine coincide with the stoichiometric air-fuel ratio”, and isconfigured so as to feedback-control the air-fuel ratio of the enginebased on the air-fuel ratio feedback amount. The air-fuel ratio feedbackamount used in each of those air-fuel ratio control apparatuses is acontrol amount commonly used for all of the cylinders.

Meanwhile, in general, an electronic-fuel-injection-type internalcombustion engine has at least one fuel injection valve (39) at each ofthe cylinders or at each of intake ports communicating with therespective cylinders. Accordingly, when the characteristic/property ofthe fuel injection valve of a certain cylinder changes to acharacteristic that it injects fuel in an amount excessively larger thanan instructed fuel injection amount, only the air-fuel ratio of anair-fuel mixture supplied to that certain cylinder (the air-fuel ratioof the certain cylinder) greatly changes toward the rich side. That is,the degree of an air-fuel ratio non-uniformity among the cylinders(inter-cylinder air-fuel ratio variation; inter-cylinder air-fuel ratioimbalance) increases. In other words, there arises an imbalance among“cylinder-by-cylinder air-fuel ratios,” each of which is the air-fuelratio of the air-fuel mixture supplied to each of the cylinders.

In such a case, the average of the air-fuel ratios of the air-fuelmixtures supplied to the entire engine becomes an air-fuel ratio richerthan the stoichiometric air-fuel ratio. Accordingly, by the air-fuelratio feedback amount commonly used for all of the cylinders, theair-fuel ratio of the above-mentioned certain cylinder is changed towardthe lean side so as to come closer to the stoichiometric air-fuel ratio,and, at the same time, the air-fuel ratios of the remaining cylindersare changed toward the lean side so as to deviate from thestoichiometric air-fuel ratio. As a result, the average of the air-fuelratios of the air-fuel mixtures supplied to the entire engine becomessubstantially equal to the stoichiometric air-fuel ratio.

However, since the air-fuel ratio of the certain cylinder is still inthe rich side in relation to the stoichiometric air-fuel ratio and theair-fuel ratios of the remaining cylinders are in the lean side inrelation to the stoichiometric air-fuel ratio, combustion of theair-fuel mixture in each of the cylinders fail to become completecombustion. As a result, the amount of emissions (the amount of unburnedcombustibles and/or the amount of nitrogen oxides) discharged from eachof the cylinders increases. Therefore, even when the average of theair-fuel ratios of the air-fuel mixtures supplied to the cylinders ofthe engine is equal to the stoichiometric air-fuel ratio, the increasedemissions cannot be completely removed by the three-way catalyst.Consequently, the amount of emissions may increase.

Accordingly, in order to prevent emissions from increasing, it isimportant to detect a state in which the degree of the air-fuel rationon-uniformity among the cylinders becomes excessively large (generationof an inter-cylinder air-fuel ratio imbalance state) for taking somemeasures against the imbalance state. It should be noted that, theinter-cylinder air-fuel ratio imbalance also occurs, for example, in acase where the characteristic of the fuel injection valve of the certaincylinder changes to a characteristic that it injects fuel in an amountexcessively smaller than the instructed fuel injection amount.

One of such conventional apparatuses for determining whether or not aninter-cylinder air-fuel ratio imbalance state has occurred is configuredso as to obtain a trace/trajectory length of an output value (outputsignal) of an air-fuel ratio sensor (the above-mentioned upstreamair-fuel ratio sensor 67) disposed at an exhaust merging/aggregatedregion/portion into which exhaust gases from a plurality of cylinders ofan engine merge, compare the trace length with a “reference value whichchanges in accordance with the rotational speed of the engine,” anddetermine whether or not the inter-cylinder air-fuel ratio imbalancestate has occurred based on the result of the comparison (see, forexample, U.S. Pat. No. 7,152,594).

It should be noted that, in the present specification, the expression of“an inter-cylinder air-fuel ratio imbalance state (excessiveinter-cylinder air-fuel ratio imbalance state)” means a state in whichthe difference between the cylinder-by-cylinder air-fuel ratios is equalto or greater than an allowable value; in other words, it means aninter-cylinder air-fuel ratio imbalance state in which the amount ofunburned combustibles and/or nitrogen oxides exceeds a prescribed value.The determination as to whether or not an “inter-cylinder air-fuel ratioimbalance state” has occurred will be simply referred to as an“inter-cylinder air-fuel ratio imbalance determination” or an “imbalancedetermination.” Moreover, a cylinder supplied with an air-fuel mixturewhose air-fuel ratio deviates from the air-fuel ratio of air-fuelmixtures supplied to the remaining cylinders (for example, an air-fuelratio approximately equal to the stoichiometric air-fuel ratio) willalso be referred to as an “imbalanced cylinder.” The air-fuel ratio ofthe air-fuel mixture supplied to such an imbalanced cylinder will alsobe referred to as an “air-fuel ratio of the imbalanced cylinder.” Theremaining cylinders (cylinders other than the imbalanced cylinder) willalso be referred to as “normal cylinders” or “balanced cylinders.” Theair-fuel ratio of air-fuel mixtures supplied to such normal cylinderswill also be referred as an “air-fuel ratio of the normal cylinders” oran “air-fuel ratio of the balanced cylinders.”

In addition, a parameter (e.g., the above-mentioned trace length of theoutput value of the air-fuel ratio sensor), whose absolute valueincreases as the difference between the cylinder-by-cylinder air-fuelratios (the difference between the air-fuel ratio of the imbalancedcylinder and those of the normal cylinders) becomes larger will also bereferred to as an “air-fuel ratio fluctuation indicating amount.” Thatis, the air-fuel ratio fluctuation indicating amount is a “valueobtained based on the output value of the air-fuel ratio sensor” in sucha manner that its absolute value becomes larger as the air-fuel ratiovariation/fluctuation of the exhaust gas reaching the above-mentionedair-fuel ratio sensor becomes larger. Further, a value, which isobtained based on the air-fuel ratio fluctuation indicating amount, andwhich becomes larger as the absolute value of the air-fuel ratiofluctuation indicating amount becomes larger, will also be referred toas an “imbalance determination parameter.” In other words, the imbalancedetermination parameter is a parameter which becomes larger as thefluctuation/variation of the air-fuel ratio of the exhaust gas passingthrough the position at which the air-fuel ratio sensor is disposedbecomes larger. This imbalance determination parameter is compared withthe imbalance determination threshold in order to perform (carry out)the imbalance determination.

SUMMARY OF THE INVENTION

As shown in (A) of FIG. 2, for example, a well-known air-fuel ratiosensor includes an air-fuel ratio detecting section, which includes atleast a solid electrolyte layer (671), an exhaust-gas-side electrodelayer (672), an atmosphere-side electrode layer (673), a diffusionresistance layer (674), and a heater (678).

The exhaust-gas-side electrode layer (672) is formed on one of surfacesof the solid electrolyte layer (671). The exhaust-gas-side electrodelayer (672) is covered with the diffusion resistance layer (674).Exhaust gas within an exhaust passage reaches an outer surface of thediffusion resistance layer (674), and reaches the exhaust-gas-sideelectrode layer (672) after passing through the diffusion resistancelayer (674). The atmosphere-side electrode layer (673) is formed on theother one of surfaces of the solid electrolyte layer (671). Theatmosphere-side electrode layer (673) is exposed to an atmospherechamber (67A) into which atmospheric air is introduced. The heater (678)generates a heat when energized so as to adjust a temperature of asensor element section. The sensor element section includes at least thesolid electrolyte layer (671), the exhaust-gas-side electrode layer(672), and the atmosphere-side electrode layer (673).

As shown in (B) and (C) of FIG. 2, a voltage (Vp) is applied between theexhaust-gas-side electrode layer (672) and the atmosphere-side electrodelayer (673) so as to generate a “limiting current which changes inaccordance with the air-fuel ratio of the exhaust gas.” In general, thisvoltage is applied such that the potential of the atmosphere-sideelectrode layer (673) becomes higher than that of the exhaust-gas-sideelectrode layer (672).

As shown in (B) of FIG. 2, when an excessive amount of oxygen iscontained in the exhaust gas reaching the exhaust-gas-side electrodelayer (672) through the diffusion resistance layer (674) (that is, whenthe air-fuel ratio of the exhaust gas reaching the exhaust-gas-sideelectrode layer is leaner than the stoichiometric air-fuel ratio), theoxygen is led in the form of oxygen ion from the exhaust-gas-sideelectrode layer (672) to the atmosphere-side electrode layer (673) owingto the application of the above-mentioned voltage and an oxygen pumpcharacteristic of the solid electrolyte layer (671).

In contrast, as shown in (C) of FIG. 2, when excessive unburnedcombustibles are contained in the exhaust gas reaching theexhaust-gas-side electrode layer (672) through the diffusion resistancelayer (674) (that is, when the air-fuel ratio of the exhaust gasreaching the exhaust-gas-side electrode layer is richer than thestoichiometric air-fuel ratio), oxygen within the atmosphere chamber(67A) is led in the form of oxygen ion from the atmosphere-sideelectrode layer (673) to the exhaust-gas-side electrode layer (672)owing to an oxygen cell characteristic of the solid electrolyte layer(671), so as to react with the unburned combustibles at theexhaust-gas-side electrode layer (672).

Because of the presence of the diffusion resistance layer (674), amoving amount of such oxygen ions is limited to a value corresponding tothe “air-fuel ratio of the exhaust gas reaching the outer surface of thediffusion resistance layer (674).” In other words, a current generatedas a result of movement of the oxygen ions has a magnitude correspondingto the air-fuel ratio (A/F) of the exhaust gas (that is, limitingcurrent Ip) (see FIG. 3).

The air-fuel ratio sensor outputs an output value Vabyfs correspondingto the “air-fuel ratio of the exhaust gas passing through the positionat which the air-fuel ratio sensor is disposed” based on the limitingcurrent (the current flowing through the solid electrolyte layer owingto the application of the voltage between the exhaust-gas-side electrodelayer and the atmosphere-side electrode layer). This output value Vabyfsis generally converted into a detected air-fuel ratio abyfs based on apreviously obtained “relationship between the output value Vabyfs andthe air-fuel ratio, shown in FIG. 4.” As understood from FIG. 4, theoutput value Vabyfs is substantially proportional to the detectedair-fuel ratio abyfs.

Meanwhile, the air-fuel ratio fluctuation indicating amount which is a“base data for the imbalance determination parameter” is not limited tothe trace length of “the output value Vabyfs of the air-fuel ratiosensor or the detected air-fuel ratio abyfs,” but may be any one ofvalues which reflect a fluctuation of the air-fuel ratio of the exhaustgas flowing through the position at which the air-fuel ratio sensor isdisposed (e.g., a fluctuation amount of one of those per/for apredetermined period). This point will be described further.

Exhaust gases from the cylinders successively reach the air-fuel ratiosensor in the order of ignition (accordingly, in the order of exhaust).In a case where no inter-cylinder air-fuel ratio imbalance state hasbeen occurring, the air-fuel ratios of the exhaust gases discharged fromthe cylinders are approximately equal to one another. Accordingly, inthe case where no inter-cylinder air-fuel ratio imbalance state has beenoccurring, as shown by a broken line C1 in (B) of FIG. 5, the waveformof the output value Vabyfs of the air-fuel ratio sensor (in (B) of FIG.5, the waveform of the detected air-fuel ratio abyfs) is almost flat.

In contrast, in a case where there has been occurring an inter-cylinderair-fuel ratio imbalance state in which only the air-fuel ratio of aspecific cylinder (for example, the first cylinder) has deviated towardthe rich side from the stoichiometric air-fuel ratio (specific-cylinderrich-side-deviated imbalance state), the air-fuel ratio of the exhaustgas from the specific cylinder greatly differs from those of the exhaustgases from the cylinders (the remaining cylinders) other than thespecific cylinder.

Accordingly, as shown by a solid line C2 in (B) of FIG. 5, the waveformof the output value Vabyfs of the air-fuel ratio sensor (in (B) of FIG.5, the waveform of the detected air-fuel ratio abyfs) in a case wherethe specific-cylinder rich-side-deviated imbalance state has beenoccurring greatly fluctuates. Specifically, in a case of afour-cylinder, four-cycle engine, the waveform of the output valueVabyfs of the air-fuel ratio sensor greatly fluctuates every 720° crankangle (the crank angle required for all of the cylinders, each of whichdischarges exhaust gas which reaches a single air-fuel ratio sensor, tocomplete their single-time combustion strokes). It should be noted that,in the present specification, a “period corresponding to the crank anglerequired for all of the cylinders, each of which discharges the exhaustgas which reaches the single air-fuel ratio sensor, to complete theirsingle-time combustion strokes” will also be referred to as a “unitcombustion cycle period.”

Further, an amplitude of the output value Vabyfs of the air-fuel ratiosensor and that of the detected air-fuel ratio abyfs become larger, asthe air-fuel ratio of the imbalanced cylinder deviates more greatly fromthe air-fuel ratios of the balanced cylinders. For example, assumingthat the detected air-fuel ratio abyfs varies as shown by a solid lineC2 in (B) of FIG. 5 when a difference between the air-fuel ratio of theimbalanced cylinder and the air-fuel ratios of the balanced cylinders isequal to a first value, the detected air-fuel ratio abyfs varies asshown by a broken line C2 a in (B) of FIG. 5 when the difference betweenthe air-fuel ratio of the imbalanced cylinder and the air-fuel ratios ofthe balanced cylinders is equal to a “second value larger than the firstvalue.”

Accordingly, a change amount per unit time of the output value Vabyfs ofthe air-fuel ratio sensor or of the detected air-fuel ratio abyfs (i.e.,a first order differential value of the output value Vabyfs of theair-fuel ratio sensor or of the detected air-fuel ratio abyfs withrespect to time, refer to angles α1, α2 shown in (B) of FIG. 5)fluctuates slightly as shown by a broken line C3 in (C) of FIG. 5 whenthe cylinder-by-cylinder air-fuel ratio difference is small, andfluctuates greatly as shown by a solid line C4 in (C) of FIG. 5 when thecylinder-by-cylinder air-fuel ratio difference is large. That is, anabsolute value of the differential value d(Vabyfs)/dt or of thedifferential value d(abyfs/dt) becomes larger as the degree of theinter-cylinder air-fuel-ratio imbalance state becomes larger (as thecylinder-by-cylinder air-fuel ratio difference becomes larger).

In view of the above, for example, “a maximum value or a mean value” ofthe absolute values of “the differential values d(Vabyfs)/dt or thedifferential values d(abyfs/dt)”, that are obtained a plurality of timesin the unit combustion cycle period can be adopted as the air-fuel ratiofluctuation indicating amount. Further, the air-fuel ratio fluctuationindicating amount itself or a mean value of the air-fuel ratiofluctuation indicating amounts obtained for a plurality of the unitcombustion cycle periods can be adopted as the imbalance determinationparameter.

Further, as shown in (D) of FIG. 5, a change amount of the change amountof “the output value Vabyfs of the air-fuel ratio sensor or of thedetected air-fuel ratio abyfs” (i.e., a second order differential valued²(Vabyfs)/dt² or a second order differential value d²(abyfs)/dt²)hardly fluctuates as shown by a broken line C5 when thecylinder-by-cylinder air-fuel ratio difference is small, but greatlyfluctuates as shown by a solid line C6 when the cylinder-by-cylinderair-fuel ratio difference is large.

In view of the above, for example, “a maximum value or a mean value” ofthe absolute values of “the second order differential valuesd²(Vabyfs)/dt² or the second order differential values d²(abyfs)/dt²”,that are obtained a plurality of times in the unit combustion cycleperiod can also be adopted as the air-fuel ratio fluctuation indicatingamount. Further, the air-fuel ratio fluctuation indicating amount itselfor a mean value of the air-fuel ratio fluctuation indicating amountsobtained for a plurality of the unit combustion cycle periods can beadopted as the imbalance determination parameter.

The inter-cylinder air-fuel ratio imbalance determination apparatusdetermines whether or not the inter-cylinder air-fuel-ratio imbalancestate has been occurring by determining whether or not the imbalancedetermination parameter thus obtained is larger than a predeterminedthreshold (imbalance determination threshold).

However, the present inventor(s) has/have acquired findings/knowledgethat a state occurs in which the output value Vabyfs of the air-fuelratio sensor fails to change/vary with respect to the fluctuation of theexhaust gas while showing a good responsiveness (or in which theresponsiveness of the air-fuel sensor is not sufficient), and in such astate, the imbalance determination parameter obtained according to theair-fuel ratio fluctuation indicating amount fails to represent the“degree of the inter-cylinder air-fuel ratio imbalance state”, and thus,the inter-cylinder air-fuel ratio imbalance determination cannot beperformed accurately.

The state in which the output value Vabyfs of the air-fuel ratio sensorfails to change/vary with respect to the fluctuation of the exhaust gaswhile showing a good responsiveness (in other words, the state in whichthe responsiveness of the air-fuel sensor becomes worse) occurs, when,for example, the air-fuel ratio of the exhaust gas fluctuates in anair-fuel ratio range which is very close to the stoichiometric air-fuelratio. It is inferred that the reason why the responsiveness of theair-fuel sensor becomes worse when the air-fuel ratio of the exhaust gasfluctuates in the air-fuel ratio range which is very close to thestoichiometric air-fuel ratio is a direction of a reaction(oxidation-reduction reaction) at the exhaust-gas-side electrode layermust change to a reverse direction when the air-fuel ratio of theexhaust gas changes from an “air-fuel ratio richer than thestoichiometric air-fuel ratio” to an “air-fuel ratio leaner than thestoichiometric air-fuel ratio,” or vice versa, and accordingly, itrequires a considerable time for a direction of the oxygen ions passingthrough the solid electrolyte layer to be reversed.

Meanwhile, FIG. 6 is a graph showing a relation between the temperatureof the element section of the air-fuel ratio sensor (hereinafter, alsoreferred to as “an air-fuel ratio sensor element temperature or a sensorelement temperature”) and the responsiveness of the air-fuel ratiosensor. In FIG. 6, a response time t representing the responsiveness ofthe air-fuel ratio sensor is, for example, a time (duration) from a“specific point in time” at which an “air-fuel ratio of the exhaust gaswhich is present in the vicinity of the air-fuel ratio sensor” ischanged from a “first air-fuel ratio (e.g., 14) richer than thestoichiometric air-fuel ratio” to a “second air-fuel ratio (e.g., 15)leaner than the stoichiometric air-fuel ratio” to a point in time atwhich the detected air-fuel ratio abyfs changes from the first air-fuelratio to a third air-fuel ratio which is between the first air-fuelratio and the second air-fuel ratio (e.g., the third air-fuel ratiobeing 14.63=14+0.63·(15−14)). Accordingly, the responsiveness of theair-fuel ratio sensor is better (higher) as the response time t isshorter.

As understood from FIG. 6, the responsiveness of the air-fuel ratiosensor is better as the air-fuel ratio sensor element temperature ishigher. It is inferred that the reason for that is the reaction(oxidation-reduction reaction) at the sensor element section(especially, at the exhaust-gas-side electrode layer) becomes moreactive. Accordingly, adjusting a heat amount of the heater in such amanner that the air-fuel ratio sensor element temperature is maintainedat a high temperature enables to obtain the imbalance determinationparameter having a high accuracy. On the other hand, if the air-fuelratio sensor element temperature is always maintained at the hightemperature, the air-fuel ratio sensor may deteriorate (deteriorationwith age may occurs) relatively early.

In view of the above, one of objects of the present invention is toprovide an inter-cylinder air-fuel ratio imbalance determinationapparatus (hereinafter, also referred to as a “present inventionapparatus”) which can accurately perform an inter-cylinder air-fuelratio imbalance determination while avoiding the deterioration of theair-fuel ratio sensor as much as possible.

The present invention apparatus controls the heater (controls an amountof heat generation) in such a manner that “an air-fuel ratio sensorelement temperature during (while) the imbalance determination parameteris being obtained (parameter-obtaining-period-element-temperature)”becomes/is higher than “an air-fuel ratio sensor element temperatureduring (while) the imbalance determination parameter is not beingobtained (parameter-non-obtaining-period-element-temperature).” Thismakes it possible to obtain the imbalance determination parameter in a“state where the responsiveness of the air-fuel ratio sensor is good.”Accordingly, the thus obtained imbalance determination parameter becomesa value which accurately represents the inter-cylinder air-fuel-ratioimbalance state (cylinder-by-cylinder air-fuel ratio difference).Consequently, the inter-cylinder air-fuel-ratio imbalance determinationcan be performed accurately.

Further, the present invention apparatus maintains the air-fuel ratiosensor element temperature for a period in which the imbalancedetermination parameter is not being obtained (theparameter-non-obtaining-period-element-temperature) at a “relatively lowtemperature which is equal to or higher than an activating temperatureof the air-fuel ratio sensor.” Accordingly, it is possible to avoid thatthe deterioration of the sensor occurs early, as compared to a case inwhich the air-fuel ratio sensor element temperature is always maintainedat a relatively high temperature.

Specifically, one of aspects of the present invention apparatus isapplied to a multi-cylinder internal combustion engine, and includes aplurality of fuel injection valves (injectors), heater control means,and imbalance determining means.

The air-fuel ratio sensor is disposed in an exhaust merging portion ofan exhaust passage of the engine into which exhaust gases dischargedfrom at least two or more (preferably, three or more) of the cylindersamong a plurality of cylinders merge, or is disposed in the exhaustpassage at a position/location downstream of the exhaust mergingportion.

Further, the air-fuel ratio sensor includes an air-fuel ratio detectingsection having a solid electrolyte layer, an exhaust-gas-side electrodelayer formed on one of surfaces of the solid electrolyte layer, adiffusion resistance layer which covers the exhaust-gas-side electrodelayer and at which the exhaust gases arrive, an atmosphere-sideelectrode layer which is formed on the other one of the surfaces of thesolid electrolyte layer and is exposed to an atmosphere chamber, and aheater. The heater heats up the sensor element section so as tocontrol/adjust a temperature of the sensor element section. The sensorelement section includes the solid electrolyte layer, theexhaust-gas-side electrode layer, and the atmosphere-side electrodelayer. In addition, the air-fuel ratio sensor outputs an output valuecorresponding to an “air-fuel ratio of the exhaust gas passing throughthe position at which the air-fuel ratio sensor is disposed” based on alimiting current flowing through the solid electrolyte layer owing to anapplication of a voltage between the exhaust-gas-side electrode layerand the atmosphere-side electrode layer.

Each of a plurality of the fuel injection valves is disposed in such amanner that each of the injection valves corresponds to each of theabove-mentioned at least two or more of the cylinders, and injects fuelcontained in an air-fuel mixture supplied to a combustion chamber of thecorresponding cylinder. That is, one or more fuel injection valves areprovided for each cylinder. Each of the fuel injection valves injectsfuel to the cylinder corresponding to that fuel injection valve.

The heater control means controls an amount of heat generation of theheater.

The imbalance determining means:

(1) obtains, based on the output value of the air-fuel ratio sensor, animbalance determination parameter which becomes larger as a“variation/fluctuation of the air-fuel ratio of the exhaust gaspassing/flowing through the position at which the air-fuel ratio sensoris disposed” becomes larger, in a “parameter obtaining period” which isa “period for/in which a predetermined parameter obtaining condition isbeing satisfied”;(2) determines that an inter-cylinder air-fuel ratio imbalance state hasoccurred, when the obtained imbalance determination parameter is largerthan a predetermined imbalance determination threshold; and(3) determines that an inter-cylinder air-fuel ratio imbalance state hasnot occurred, when the obtained imbalance determination parameter issmaller than the imbalance determination threshold.

The imbalance determination parameter may be, for example, one of “amaximum value or a mean value” of absolute values of “the abovementioned differential values d(Vabyfs)/dt or of the above mentioneddifferential values d(abyfs/dt)” for a predetermined period (e.g., forthe unit combustion cycle period); “a maximum value or a mean value” ofthe absolute values of “the second order differential valuesd²(Vabyfs)/dt² or of the second order differential value d²((abyfs)/dt²”for a predetermined period (e.g., for the unit combustion cycle period);a trace length and the like of “the output value Vabyfs or the detectedair-fuel ratio abyfs”; and a value based on one of those values. Theimbalance determination parameter is not limited to those values.

Further, the imbalance determining means is configured so as to make the“heater control means” perform a control to have an “air-fuel ratiosensor element temperature(parameter-obtaining-period-element-temperature) for aparameter-obtaining-period” be higher than an “air-fuel ratio sensorelement temperature (parameter-non-obtaining-period-element-temperature)for a period (parameter-non-obtaining-period) other than theparameter-obtaining-period.” The control is also referred to as a“sensor element section temperature elevating control”. In other words,the parameter-non-obtaining-period-element-temperature is set at (to) a“first temperature”, and the“parameter-obtaining-period-element-temperature” is set at (to) a“second temperature higher than the first temperature.”

According to the above configuration, the imbalance determinationparameter is obtained when the responsiveness of the air-fuel ratiosensor is good by raising (elevation of) the temperature of the air-fuelratio sensor element (i.e., when the output value of the air-fuel ratiosensor can follow the fluctuation of the air-fuel ratio of the exhaustgas without an excessive delay). Accordingly, since the imbalancedetermination parameter becomes a value which can accurately representthe cylinder-by-cylinder air-fuel ratio difference, it can be determinedaccurately whether or not the inter-cylinder air-fuel-ratio imbalancestate has been occurring.

Further, according to the above configuration, the temperature of theair-fuel ratio sensor element during the parameter non-obtaining periodis controlled to the relatively low temperature (the first temperature).Accordingly, it is possible to avoid that the sensor deteriorates early(early deterioration with age) due to heat, as compared to the case inwhich the air-fuel ratio sensor element temperature is always maintainedat the relatively high temperature (the second temperature).

It should be noted that the parameter obtaining condition may include,for example, at least one or more of following conditions.

-   -   The imbalance determination has never performed since the        current start of the engine.    -   An intake air flow rate is within a predetermined range.    -   An engine rotational speed is within a predetermined range.    -   A cooling water temperature is equal to or higher than a cooling        water temperature threshold.    -   A predetermined time has elapsed since a point in time at which        a change amount of “a throttle valve opening or an operation        amount of an accelerator pedal” per unit time becomes equal to        or smaller than a predetermined value.

The parameter obtaining condition is not limited to those.

Meanwhile, when the cylinder-by-cylinder air-fuel ratio difference isvery large, the air-fuel ratio of the exhaust gas fluctuates extremelygreatly. Accordingly, when the cylinder-by-cylinder air-fuel ratiodifference is very large, the obtained imbalance determination parameterbecomes extremely large even if the responsiveness of the air-fuel ratiosensor is relatively low. It is therefore possible to clearly determinethat the inter-cylinder air-fuel-ratio imbalance state has beenoccurring, when the imbalance determination parameter is obtained whilethe air-fuel ratio sensor element temperature is maintained at therelatively low temperature (the first temperature), and thus obtainedimbalance determination parameter is larger than a “predeterminedthreshold (also referred to as a high-side threshold).”

In contrast, when the cylinder-by-cylinder air-fuel ratio difference isvery small, the air-fuel ratio of the exhaust gas fluctuates extremelyslightly. Accordingly, even if the imbalance determination parameter isobtained when the responsiveness of the air-fuel ratio sensor isrelatively low, it is possible to clearly determine that theinter-cylinder air-fuel-ratio imbalance state has not been occurring ifthe obtained imbalance determination parameter is extremely small. Inother words, when the imbalance determination parameter is obtainedwhile the air-fuel ratio sensor element temperature is maintained at therelatively low temperature (the first temperature), and thus obtainedimbalance determination parameter is smaller than a “predeterminedthreshold (also referred to as a low-side threshold) which is smaller bya predetermined value than the high-side threshold”, it is possible toclearly determine that the inter-cylinder air-fuel-ratio imbalance statehas not been occurring.

In view of the above, the imbalance determining means of another aspectof the present invention apparatus is configured so as to:

(4) obtain, based on the output value of the air-fuel ratio sensor, theimbalance determination parameter as a tentative parameter before havingthe heater control means perform the sensor element section temperatureelevating control (i.e., while maintaining the air-fuel ratio sensorelement temperature at the relatively low temperature) in/during theparameter obtaining period;(5) determine that the inter-cylinder air-fuel ratio imbalance state hasbeen occurring, when the obtained tentative parameter is larger than thepredetermined high-side threshold; and(6) determine that the inter-cylinder air-fuel ratio imbalance state hasnot been occurring, when the obtained tentative parameter is smallerthan the “low-side threshold which is smaller by the predetermined valuethan the high-side threshold.”

In this case, it is preferable that the high-side threshold be a valuewhich is equal to or larger than the imbalance determination threshold,and the low-side threshold be a value which is equal to or smaller thanthe imbalance determination threshold.

To the contrary, when the imbalance determination parameter obtainedwhile the air-fuel ratio sensor element temperature is relatively low(while the responsiveness of the air-fuel ratio sensor is relativelylow) is between the high-side threshold and the low-side threshold, itis not possible to clearly determine whether or not the inter-cylinderair-fuel-ratio imbalance state has occurred.

Accordingly, the imbalance determining means of another aspect of thepresent invention apparatus is configured so as to:

(7) withhold (making) the determination as to whether or not theinter-cylinder air-fuel-ratio imbalance state has occurred, when theobtained tentative parameter is between the high-side threshold and thelow-side threshold;(8) have the heater control means perform the sensor element sectiontemperature elevating control during the parameter obtaining period, andobtain, based on the output value of the air-fuel ratio sensor, theimbalance determination parameter as a final parameter, while (in thecase in which) the determination as to whether or not the inter-cylinderair-fuel-ratio imbalance state has occurred is being withheld; and(9) determine that the inter-cylinder air-fuel-ratio imbalance state hasoccurred when the obtained final parameter is larger than the imbalancedetermination threshold, and determine that the inter-cylinderair-fuel-ratio imbalance state has not occurred when the obtained finalparameter is smaller than the imbalance determination threshold.

According to the above configuration, in the case in which thedetermination as to whether or not the inter-cylinder air-fuel-ratioimbalance state has occurred is withheld, the air-fuel ratio sensorelement temperature is elevated (raised), and thus, the imbalancedetermination parameter (the final parameter) can be obtained while theresponsiveness of the air-fuel ratio sensor is high. Accordingly, evenin the case in which it is not possible to clearly determine whether ornot the inter-cylinder air-fuel-ratio imbalance state has occurred usingthe tentative parameter, the imbalance determination can be performedaccurately using the final parameter.

Further, according to the apparatus of the above aspect, it is notnecessary to perform the sensor element section temperature elevatingcontrol in the case in which it is possible to clearly determine whetheror not the inter-cylinder air-fuel-ratio imbalance state has occurredusing the imbalance determination parameter (the tentative parameter)obtained while the responsiveness of the air-fuel ratio sensor isrelatively low. Accordingly, since chances/frequency that the air-fuelratio sensor element temperature is elevated up to the relatively hightemperature for the imbalance determination decreases, it can be avoidedthat the deterioration of the air-fuel ratio sensor is accelerated.

It requires some time for the air-fuel ratio sensor element temperatureto actually becomes higher after a start of the execution of the sensorelement section temperature elevating control. Accordingly, if theimbalance determination parameter is obtained immediately after thestart of the execution of the sensor element section temperatureelevating control, the imbalance determination parameter may be obtainedwhile the responsiveness of the air-fuel ratio sensor is notsufficiently high.

In view of the above, in the present invention apparatus configured soas to perform the sensor element section temperature elevating control,it is preferable that the imbalance determining means be configured soas to start to obtain the imbalance determination parameter after apredetermined delay time has elapsed since a point in time at which thesensor element section temperature elevating control was started.

According to the above configuration, the imbalance determinationparameter can be obtained based on the output value of the air-fuelratio sensor after a point in time at which the responsiveness of theair-fuel ratio sensor becomes sufficiently high owing to the elevation(high temperature) of the air-fuel ratio sensor element temperature. Itis therefore possible to obtain the imbalance determining parameterwhich more accurately represents the cylinder-by-cylinder air-fuel ratiodifference.

It is preferable that the imbalance determining means be configured soas to set the predetermined delay time in such a manner that the delaytime is shorter as a temperature of the exhaust gas is higher.

It is also preferable that the imbalance determining means be configuredso as to set the predetermined delay time in such a manner that thedelay time is shorter as “the intake air flow rate of the engine or theload of the engine” is greater.

The air-fuel ratio sensor element temperature increases more rapidly asthe temperature of the exhaust gas is higher. Accordingly, the delaytime can be (set) shorter as the temperature of the exhaust gas ishigher. The temperature of the exhaust gas may be obtained from anexhaust gas temperature sensor, or may be estimated based on the intakeair flow rate or the load of the engine. In this case, the temperatureof the exhaust gas becomes higher as the intake air flow rate or theload of the engine becomes greater. Accordingly, the delay time can be(set) shorter as the intake air flow rate or the load of the engine isgreater.

As described before, it requires some time for the air-fuel ratio sensorelement temperature to actually increase after the start of theexecution of the sensor element section temperature elevating control.Accordingly, if the sensor element section temperature elevating controlis started after the parameter obtaining condition becomes satisfied,there may be a case in which obtaining the imbalance determinationparameter can not be started until the air-fuel ratio sensor elementtemperature becomes sufficiently high. In addition, if the parameterobtaining condition becomes unsatisfied in a period from the start ofthe execution of the sensor element section temperature elevatingcontrol to a point in time at which the air-fuel ratio sensor elementtemperature becomes sufficiently high, the sensor element sectiontemperature elevating control is stopped. Consequently,chances/frequency to obtain the imbalance determination parameter maydecrease.

On the other hand, in a case in which the engine has not been warmed upyet since the start of the engine, moisture in the exhaust gas is cooleddown to form water droplets. In such a case in which it is likely thatthe water droplets adhere to the air-fuel ratio detecting section of theair-fuel ratio sensor (hereinafter, this is expressed as “the air-fuelratio sensor gets wet with water”), if the temperature of the “air-fuelratio detecting section including the sensor element section” iselevated by the sensor element section temperature elevating control, agreat temperature unevenness in the air-fuel ratio detecting sectionoccurs when the air-fuel ratio sensor actually get wet with water, andthus, the air-fuel ratio detecting section may crack/dunt (be broken).Accordingly, it is not preferable to perform the sensor element sectiontemperature elevating control immediately after the start of the engine.

In view of the above, the imbalance determining means of another aspectof the present invention apparatus is configured so as to have theheater control means start to perform the sensor element sectiontemperature elevating control at a point in time at which the warming-upof the engine is completed after the start of the engine, andfinishes/ends the sensor element section temperature elevating controlat a point in time at which obtaining the imbalance determinationparameter is completed.

It is unlikely that the air-fuel ratio sensor gets wet with water aftera point in time at which the warming-up of the engine is completed.Accordingly, if the sensor element section temperature elevating controlis started at the point in time at which the warming-up of the engine iscompleted, it is unlikely that the air-fuel ratio sensor gets wet withwater. In addition, according to the above configuration,changces/frequency that the air-fuel ratio sensor element temperaturehas become sufficiently high at the point in time at which the parameterobtaining condition becomes satisfied can be increased,changces/frequency that the imbalance determination parameter having ahigh accuracy is obtained can be increased.

A temperature of the solid electrolyte layer which is a part of thesensor element section of the air-fuel ratio sensor has a strongcorrelation with an admittance (inverse of the impedance) of the solidelectrolyte layer. In general, the admittance of the solid electrolytelayer becomes higher as the temperature of the solid electrolyte layerbecomes higher.

In view of the above, the heater control means is configured so as tocontrol the amount of heat generation of the heater in such a mannerthat a difference between a value corresponding to the actual admittanceof the solid electrolyte layer (e.g., the admittance or the impedance)and a target value is decreased, and so as to realize the sensor elementsection temperature elevating control by making the target value duringthe sensor element section temperature elevating control is beingperformed different from the target value during the sensor elementsection temperature elevating control is not being performed.

For example, the “value corresponding to the actual admittance of thesolid electrolyte layer” is the actual admittance of the solidelectrolyte layer, the target value during the sensor element sectiontemperature elevating control is being performed is made higher than thetarget value during the sensor element section temperature elevatingcontrol is not being performed. Alternatively, the “value correspondingto the actual admittance of the solid electrolyte layer” is the actualimpedance of the solid electrolyte layer, the target value during thesensor element section temperature elevating control is being performedis made lower than the target value during the sensor element sectiontemperature elevating control is not being performed.

Meanwhile, the air-fuel ratio sensor changes with age (the passage oftime) when the air-fuel ratio is used for a long time. Consequently, asshown in FIG. 23, the admittance (refer to a broken line Y2) of theair-fuel ratio sensor which has changed with the passage of time issmaller than the admittance (refer to a solid line Y1) of the air-fuelratio sensor which has not changed with the passage of time.

Accordingly, even when the actual admittance of the solid electrolytelayer coincides with a “certain specific admittance”, the air-fuel ratiosensor element temperature when the air-fuel ratio sensor has notchanged with the passage of time is higher than the air-fuel ratiosensor element temperature when the air-fuel ratio sensor has changedwith the passage of time. In other words, in a case in which the heatercontrol is performed based on the admittance and the air-fuel ratiosensor has changed with the passage of time, the air-fuel ratio sensorelement temperature is sufficiently high and the responsiveness of theair-fuel ratio sensor is good even when the target value (targetadmittance) during the sensor element section temperature elevatingcontrol is being performed is not made higher than the target value(target admittance) during the sensor element section temperatureelevating control is not being performed. Similarly, in a case in whichthe heater control is performed based on the impedance and the air-fuelratio sensor has changed with the passage of time, the air-fuel ratiosensor element temperature is sufficiently high and the responsivenessof the air-fuel ratio sensor is good even when the target value (targetimpedance) during the sensor element section temperature elevatingcontrol is being performed is not made lower than the target value(target impedance) during the sensor element section temperatureelevating control is not being performed.

In view of the above, it is preferable that the imbalance determiningmeans be configured so as to include deterioration-with-age-occurrencedetermining means for determining whether or not the air-fuel ratiosensor has deteriorated with age, and obtain, when it is determined thatthe air-fuel ratio has deteriorated with age, the imbalancedetermination parameter without performing the sensor element sectiontemperature elevating control even when the sensor element sectiontemperature elevating control should be performed.

According to the above configuration, since the air-fuel ratio sensorelement temperature is not elevated more than necessary, it is possibleto avoid that the deterioration of the sensor occurs early.

Another aspect of the determination apparatus according to the presentinvention is applied to the multi-cylinder internal combustion engine,and includes the air-fuel ratio sensor, and a plurality of the fuelinjection valves (injectors), similarly to the above mentioned aspect,and further includes heater control means configured as follows.

That is, the imbalance determining means is configured so as to:

(10) control the temperature of the sensor element section to the firsttemperature using the heater during the parameter obtaining period inwhich a predetermined parameter obtaining condition is satisfied, andobtain, as a usual temperature air-fuel ratio fluctuation indicatingamount, a value corresponding to an air-fuel ratio fluctuationindicating amount which becomes larger as a fluctuation of the air-fuelratio of said exhaust gas passing/flowing through the position at whichthe air-fuel ratio sensor is disposed becomes larger;(11) control the temperature of the sensor element section to a secondtemperature higher than the first temperature using the heater duringthe parameter obtaining period, and obtain, as an elevated temperatureair-fuel ratio fluctuation indicating amount, a value corresponding toan air-fuel ratio fluctuation indicating amount which becomes larger asthe fluctuation of the air-fuel ratio of said exhaust gaspassing/flowing through the position at which the air-fuel ratio sensoris disposed becomes larger;(12) obtain, based on the elevated temperature air-fuel ratiofluctuation indicating amount and the usual temperature air-fuel ratiofluctuation indicating amount, a value which becomes larger as a degreebecomes larger of difference between the elevated temperature air-fuelratio fluctuation indicating amount and the usual temperature air-fuelratio fluctuation indicating amount, as an imbalance determinationparameter;(13) determine that an inter-cylinder air-fuel-ratio imbalance state hasoccurred when the obtained imbalance determination parameter is largerthan a predetermined imbalance determination threshold, and determinethat an inter-cylinder air-fuel-ratio imbalance state has not occurredwhen the obtained imbalance determination parameter is smaller than thepredetermined imbalance determination threshold.

FIG. 11 is one of examples of a graph showing how the air-fuel ratiofluctuation indicating amount changes with respect to the air-fuel ratiosensor element temperature. In FIG. 11, a solid line L2 indicates theair-fuel ratio fluctuation indicating amount when the inter-cylinderair-fuel-ratio imbalance state has been occurring, and a broken line L1indicates the air-fuel ratio fluctuation indicating amount when theinter-cylinder air-fuel-ratio imbalance state has not been occurring.

As understood from FIG. 11, a value DX (e.g., DX=Ztujo−Ztup) increasesas the air-fuel ratio sensor element temperature increases, the value DXbeing a value which becomes larger as the “degree of difference betweenthe elevated temperature air-fuel ratio fluctuation indicating amountZtup and the usual temperature air-fuel ratio fluctuation indicatingamount Ztujo” becomes larger. Further, the “value DX (=DX1) when theinter-cylinder air-fuel-ratio imbalance state has been occurring (referto the solid line L2)” is larger than the “value DX (=DX2) when theinter-cylinder air-fuel-ratio imbalance state has not been occurring(refer to the broken line L1)”. Furthermore, a difference between thevalue DX1 and the value DX2 becomes larger as the air-fuel ratio sensorelement temperature (more accurately, a difference between the elevatedtemperature and the usual temperature) becomes larger.

Accordingly, as the above configuration, the imbalance determination canbe performed/made accurately, by obtaining values corresponding to theair-fuel ratio fluctuation indicating amount at the first temperatureand at the second temperature, obtaining an imbalance determinationparameter based on a value which becomes larger as a degree ofdifference between the values corresponding to the air-fuel ratiofluctuation indicating amount (e.g., a difference between the valuescorresponding to the air-fuel ratio fluctuation indicating amount or aratio of those values) becomes larger; and performing an imbalancedetermination based on the imbalance determination parameter. Further,that imbalance determination parameter is a value obtained when aneffect/impact which an individual difference among the air-fuel ratiosensors has on the imbalance determination parameter is diminished, andthe imbalance determination can therefore be performed accurately.

The air-fuel ratio detecting section of the air-fuel ratio sensorincludes a catalytic section which has an oxygen storage function andaccelerates an oxidation-reduction reaction, and

the air-fuel ratio sensor is configured so as to have the exhaust gaspassing through the exhaust passage reach the diffusion resistance layerthrough the catalytic section.

For example, when the rich-side-deviated imbalance state occurs, anaverage (mean) value of the air-fuel ratio of the exhaust gas changes toa certain rich air-fuel ratio. In this case, as compared to a case inwhich air-fuel ratios of all of the cylinders change to that certainrich air-fuel ratio, the unburned combustibles including hydrogengenerate in a greater amount. Since a particle diameter of hydrogen issmall, hydrogen can pass through the diffusion resistance layer of theair-fuel ratio detecting section more easily than the other unburnedcombustibles. As a result, the output value of the air-fuel ratio sensorshifts to a value richer than that certain rich air-fuel ratio.Consequently, the air-fuel ratio feedback control based on the outputvalue of the air-fuel ratio sensor may not be performed properly.

In contrast, when the catalytic section is provided with the air-fuelratio sensor, the excessive hydrogen can be oxidized at the catalyticsection. Accordingly, the excessive hydrogen which is contained in theexhaust gas reaching the exhaust-gas-side electrode layer can bedecreased. Consequently, the output value of the air-fuel ratio sensorcomes close to a value which represents the air-fuel ratio of theexhaust gas accurately.

However, a “change of the output value of the air-fuel ratio sensor withrespect to the change of the air-fuel ratio of the exhaust gas” due tothe oxidation-reduction reaction and the oxygen storage function isdelayed. Consequently, the responsiveness of the air-fuel ratio sensorof the air-fuel ratio sensor is lower than the responsiveness of theair-fuel ratio sensor without the catalytic section. Especially, whenthe exhaust gas fluctuates in such a manner that it crosses over thestoichiometric air-fuel ratio, the delay of the output value of theair-fuel ratio sensor due to the oxygen storage function becomesprominent (great).

Accordingly, in the case in which the air-fuel ratio sensor having thecatalytic section is used, the imbalance determination parameter becomesmuch smaller when the air-fuel ratio of the exhaust gas fluctuates inthe vicinity of the stoichiometric air-fuel ratio. Thus, in the case inwhich the imbalance determination is performed using the imbalancedetermination parameter obtained based on the output value of theair-fuel ratio sensor in the internal combustion engine having theair-fuel ratio sensor including the catalytic section, the presentinvention apparatus which obtains the imbalance determination parameterwhile the responsiveness of the air-fuel ratio sensor is improved byelevating the air-fuel ratio sensor element temperature can have a morebeneficial effect.

The air-fuel ration sensor usually further comprises a protective cover,which accommodates the air-fuel ratio detecting section so as to coverthe air-fuel ratio detecting section in its inside, and which includesan inflow hole for allowing the exhaust gas flowing through the exhaustpassage to flow into the inside and an outflow hole for allowing theexhaust gas which has flowed into the inside to flow out to the exhaustpassage.

In this case, it is preferable that the imbalance determining means beconfigured so as to obtain, as a “base indicating amount”, adifferential value of “the output value or the detected air-fuel ratiorepresented by the output value” with respect to time, and obtain theimbalance determination parameter based on the base indicating amount.

As long as the cylinder-by-cylinder air-fuel ratio difference is notequal to “0”, the output value of the air-fuel ratio sensor and thedetected air-fuel ratio fluctuates/varies in a cycle which is equal tothe unit combustion cycle. Accordingly, the trace length of the outputvalue Vabyfs is strongly affected by the engine rotational speed. It istherefore necessary to set the imbalance determination threshold inaccordance with the engine rotational speed accurately.

In contrast, in the case in which the air-fuel ratio sensor comprisesthe protective cover, a flow rate of the exhaust gas in the protectivecover does not change depending on the engine rotational speed, butchanges depending on a flow rate of the exhaust gas flowing through theexhaust passage (accordingly, the intake air flow rate). This is becausethe exhaust gas is flowed into the inside of the protective coverthrough the intake hole of the protective cover owing to a negativepressure flowing in the vicinity of the outflow hole of the protectivecover.

Accordingly, as long as the intake air flow rate is constant, “thedifferential value d(Vabyfs/dt) of the output value of the air-fuelratio sensor with respect to time, or the differential value d(abyfs/dt)of the detected air-fuel ratio represented by the output value of theair-fuel ratio sensor with respect to time” accurately represents thefluctuation of the air-fuel ratio of the exhaust gas, without dependingon the engine rotational speed. In view of the above, when thesedifferential values are obtained as the basic indicating amounts, andthe imbalance determination parameter is obtained based on the thusobtained differential values, the imbalance determination parameter canbe obtained as a value which can accurately represent thecylinder-by-cylinder air-fuel ratio difference regardless of whether theengine rotational speed is high or not.

Alternatively, it is preferable that the imbalance determining means beconfigured so as to obtain, as a “base indicating amount”, asecond-order differential value of “the output value or the detectedair-fuel ratio represented by the output value” with respect to time,and obtain the imbalance determination parameter based on the baseindicating amount.

The second-order differential value of the output value of the air-fuelratio sensor or of the detected air-fuel ratio represented by the outputvalue of the air-fuel ratio sensor, with respect to time (d²(Vabyfs/dt²)or d²(abyfs/dt²)) is hardly affected by a moderate/slow change of theaverage value of the air-fuel ratio of the exhaust gas. Accordingly,when these second-order differential values are obtained as the basicindicating amounts, and the imbalance determination parameter isobtained based on the thus obtained differential values, the imbalancedetermination parameter can be obtained as a “value which can accuratelyrepresent the cylinder-by-cylinder air-fuel ratio difference” even ifthe center of the air-fuel ratio of the exhaust gas moderately varies.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic plan view of an internal combustion engine towhich the inter-cylinder air-fuel ratio imbalance determinationapparatus according to each of the embodiments of the present inventionis applied.

FIG. 2 (A) to (C) of FIG. 2 are schematic sectional views of an air-fuelratio detecting section provided in an air-fuel ratio sensor (upstreamair-fuel ratio sensor) shown in FIG. 1.

FIG. 3 is a graph showing the relation between the air-fuel ratio ofexhaust gas and the limiting current of the air-fuel ratio sensor.

FIG. 4 is a graph showing the relation between the air-fuel ratio ofexhaust gas and the output value of the air-fuel ratio sensor.

FIG. 5 is a set of time charts showing behaviors of values associatedwith imbalance determination parameters for the case where aninter-cylinder air-fuel ratio imbalance state has occurred and the casewhere an inter-cylinder air-fuel ratio imbalance state has not occurred.

FIG. 6 is a graph showing the responsiveness of the air-fuel ratiosensor with respect to an air-fuel ratio sensor element temperature.

FIG. 7 is a diagram schematically showing the configuration of theinternal combustion engine shown in FIG. 1.

FIG. 8 is a partial schematic perspective view (through-view) of theair-fuel ratio sensor (upstream air-fuel ratio sensor) shown in FIGS. 1and 7.

FIG. 9 is a partial sectional view of the air-fuel ratio sensor shown inFIGS. 1 and 7.

FIG. 10 is a graph showing the relation between the air-fuel ratio ofexhaust gas and the output value of the downstream air-fuel ratio sensorshown in FIGS. 1 and 7.

FIG. 11 is a graph showing a behavior of an air-fuel ratio fluctuationindicating amount with respect to the air-fuel ratio sensor elementtemperature.

FIG. 12 is a flowchart showing a routine executed by a CPU of aninter-cylinder air-fuel ratio imbalance determination apparatus (firstdetermination apparatus) according to a first embodiment of the presentinvention.

FIG. 13 is a flowchart showing another routine executed by the CPU ofthe first determination apparatus.

FIG. 14 is a flowchart showing another routine executed by the CPU ofthe first determination apparatus.

FIG. 15 is a flowchart showing another routine executed by the CPU ofthe first determination apparatus.

FIG. 16 is a flowchart showing a routine executed by a CPU of aninter-cylinder air-fuel ratio imbalance determination apparatus (seconddetermination apparatus) according to a second embodiment of the presentinvention.

FIG. 17 is a flowchart showing another routine executed by the CPU ofthe second determination apparatus.

FIG. 18 is a flowchart showing another routine executed by the CPU ofthe second determination apparatus.

FIG. 19 is a flowchart showing a routine executed by a CPU of aninter-cylinder air-fuel ratio imbalance determination apparatus (thirddetermination apparatus) according to a third embodiment of the presentinvention.

FIG. 20 is a flowchart showing a routine executed by a CPU of aninter-cylinder air-fuel ratio imbalance determination apparatus (fourthdetermination apparatus) according to a fourth embodiment of the presentinvention.

FIG. 21 is a flowchart showing another routine executed by the CPU ofthe fourth determination apparatus.

FIG. 22 is a flowchart showing another routine executed by the CPU ofthe fourth determination apparatus.

FIG. 23 is a graph showing the relation between the air-fuel ratiosensor element temperature and the admittance of the solid electrolytelayer.

FIG. 24 is a flowchart showing a routine executed by a CPU of aninter-cylinder air-fuel ratio imbalance determination apparatus (fifthdetermination apparatus) according to a fifth embodiment of the presentinvention.

FIG. 25 is a flowchart showing another routine executed by the CPU ofthe fifth determination apparatus.

FIG. 26 is a flowchart showing a routine executed by a CPU of aninter-cylinder air-fuel ratio imbalance determination apparatus (sixthdetermination apparatus) according to a sixth embodiment of the presentinvention.

FIG. 27 is a flowchart showing another routine executed by the CPU ofthe sixth determination apparatus.

MODE FOR CARRYING OUT THE INVENTION

An inter-cylinder air-fuel ratio imbalance determination apparatus(hereinafter may be simply referred to as a “determination apparatus”)for an internal combustion engine according to each of embodiments ofthe present invention will be described with reference to the drawings.This determination apparatus is a portion of an air-fuel ratio controlapparatus for controlling the air-fuel ratio of gas mixture supplied tothe internal combustion engine (the air-fuel ratio of the engine), andalso serves as a portion of a fuel injection amount control apparatusfor controlling the amount of fuel injection.

First Embodiment Configuration

FIG. 7 schematically shows the configuration of a system configured suchthat a determination apparatus according to a first embodiment(hereinafter also referred to as a “first determination apparatus”) isapplied to a spark-ignition multi-cylinder (straight 4-cylinder)four-cycle internal combustion engine 10. Although FIG. 7 shows thecross section of a specific cylinder only, the remaining cylinders havethe same configuration.

This internal combustion engine 10 includes a cylinder block section 20including a cylinder block, a cylinder block lower-case, an oil pan,etc.; a cylinder head section 30 fixedly provided on the cylinder blocksection 20; an intake system 40 for supplying gasoline gas mixture tothe cylinder block section 20; and an exhaust system 50 for dischargingexhaust gas from the cylinder block section 20 to the exterior of theengine.

The cylinder block section 20 includes cylinders 21, pistons 22,connecting rods 23, and a crankshaft 24. Each of the pistons 22reciprocates within the corresponding cylinder 21. The reciprocatingmotion of the piston 22 is transmitted to the crankshaft 24 via therespective connecting rod 23, whereby the crankshaft 24 is rotated. Thewall surface of the cylinder 21 and the top surface of the piston 22form a combustion chamber 25 in cooperation with the lower surface ofthe cylinder head section 30.

The cylinder head section 30 includes an intake port 31 communicatingwith the combustion chamber 25; an intake valve 32 for opening andclosing the intake port 31; a variable intake timing control apparatus33 which includes an intake camshaft for driving the intake valve 32 andwhich continuously changes the phase angle of the intake camshaft; anactuator 33 a of the variable intake timing control apparatus 33; anexhaust port 34 communicating with the combustion chamber 25; an exhaustvalve 35 for opening and closing the exhaust port 34; a variable exhausttiming control apparatus 36 which includes an exhaust camshaft fordriving the exhaust valve 35 and which continuously changes the phaseangle of the exhaust camshaft; an actuator 36 a of the variable exhausttiming control apparatus 36; a spark plug 37; an igniter 38 including anignition coil for generating a high voltage to be applied to the sparkplug 37; and a fuel injection valve (fuel injection means; fuel supplymeans) 39.

The fuel injection valves (fuel injector) 39 are disposed such that asingle fuel injection valve is provided for each combustion chamber 25.The fuel injection valve 39 is provided at the intake port 31. When thefuel injection valve 39 is normal, in response to an injectioninstruction signal, the fuel injection valve 39 injects “fuel of anamount corresponding to an instructed fuel injection amount contained inthe injection instruction signal” into the corresponding intake port 31.In this way, each of a plurality of the cylinders has the fuel injectionvalve 39 which supplies fuel thereto independently of other cylinders.

The intake system 40 includes an intake manifold 41, an intake pipe 42,an air filter 43, and a throttle valve 44.

As shown in FIG. 1, the intake manifold 41 is composed of a plurality ofbranch portions 41 a and a surge tank 41 b. One end of each of aplurality of the branch portions 41 a is connected to each of aplurality of the corresponding intake ports 31, as shown in FIG. 7. Theother end of each of a plurality of the branch portions 41 a isconnected to the surge tank 41 b. One end of the intake pipe 42 isconnected to the surge tank 41 b. The air filter 43 is provided at theother end of the intake pipe 42. The throttle valve 44 is providedwithin the intake pipe 42 and adapted to change the opening crosssectional area of the intake passage. The throttle valve 44 is rotatedwithin the intake pipe 42 by a throttle valve actuator 44 a (a portionof throttle valve drive means) including a DC motor.

The exhaust system 50 includes an exhaust manifold 51, an exhaust pipe52, an upstream catalyst 53 disposed in the exhaust pipe 52, and anunillustrated downstream catalyst disposed in the exhaust pipe 52 at aposition downstream of the upstream catalyst 53.

As shown in FIG. 1, the exhaust manifold 51 has a plurality of branchportions 51 a whose one ends are connected to the exhaust ports, and amerging portion 51 b where all of the branch portions 51 a at their theother ends merge together. The merging portion 51 b is also referred toas an exhaust merging portion HK, since exhaust gases discharged from aplurality (two or more, or four in the present example) of cylindersmerge together at the merging portion 51 b. The exhaust pipe 52 isconnected to the merging portion 51 b. As shown in FIG. 7, the exhaustports 34, the exhaust manifold 51, and the exhaust pipe 52 constitute anexhaust passage.

Each of the upstream catalyst 53 and the downstream catalyst is aso-called three-way catalyst unit (exhaust purifying catalyst) carryingan active component formed of a noble metal such as platinum, rhodium,palladium, or the like. Each of the catalysts has a function ofoxidizing unburned combustibles such as HC, CO, and H₂ and reducingnitrogen oxides (NOx) when the air-fuel ratio of gas flowing into eachcatalyst coincides with the stoichiometric air-fuel ratio. This functionis also called a “catalytic function.” Furthermore, each catalyst has anoxygen storage function of occluding (storing) oxygen. This oxygenstorage function enables removal of the unburned combustibles and thenitrogen oxides even when the air-fuel ratio deviates from thestoichiometric air-fuel ratio. This oxygen storage function is realizedby ceria (CeO₂) carried by the catalyst.

This system includes a hot-wire air flowmeter 61, a throttle positionsensor 62, a water temperature sensor 63, a crank position sensor 64, anintake-cam position sensor 65, an exhaust-cam position sensor 66, anupstream air-fuel ratio sensor 67, a downstream air-fuel ratio sensor68, and an accelerator opening sensor 69.

The air flowmeter 61 outputs a signal representing the mass flow rate(intake air flow rate) Ga of an intake air flowing through the intakepipe 42. That is, the intake air flow rate Ga represents the amount ofair taken into the engine 10 per unit time.

The throttle position sensor 62 detects the opening of the throttlevalve 44 (throttle valve opening), and outputs a signal representing thedetected throttle valve opening TA.

The water temperature sensor 63 detects the temperature of cooling waterof the internal combustion engine 10, and outputs a signal representingthe detected cooling water temperature THW.

The crank position sensor 64 outputs a signal including a narrow pulsegenerated every time the crankshaft 24 rotates 10° and a wide pulsegenerated every time the crankshaft 24 rotates 360°. This signal isconverted to an engine rotational speed NE by an electric controller 70,which will be described later.

The intake-cam position sensor 65 outputs a single pulse when the intakecamshaft rotates 90 degrees from a predetermined angle, when the intakecamshaft rotates 90 degrees after that, and when the intake camshaftfurther rotates 180 degrees after that. Based on the signals from thecrank position sensor 64 and the intake-cam position sensor 65, theelectric controller 70, which will be described later, obtains theabsolute crank angle CA, while using, as a reference, the compressiontop dead center of a reference cylinder (e.g., the first cylinder). Thisabsolute crank angle CA is set to a “0° crank angle” at the compressiontop dead center of the reference cylinder, increases up to a 720° crankangle in accordance with the rotational angle of the crank angle, and isagain set to the “0° crank angle” at that point in time.

The exhaust-cam position sensor 66 outputs a single pulse when theexhaust camshaft rotates 90 degrees from a predetermined angle, when theexhaust camshaft rotates 90 degrees after that, and when the exhaustcamshaft further rotates 180 degrees after that.

As is also shown in FIG. 1, the upstream air-fuel ratio sensor 67 (anair-fuel ratio sensor in the present invention) is disposed on/in“either one of the exhaust manifold 51 and the exhaust pipe 52 (that is,the exhaust passage)” at a position between the upstream catalyst 53 andthe merging portion (exhaust merging portion HK) 51 b of the exhaustmanifold 51. The upstream air-fuel ratio sensor 67 is a“limiting-current-type wide range air-fuel ratio sensor including adiffusion resistance layer” disclosed in, for example, Japanese PatentApplication Laid-Open (kokai) Nos. H11-72473, 2000-65782, and2004-69547.

As shown in FIGS. 8 and 9, the upstream air-fuel ratio sensor 67includes an air-fuel ratio detecting section 67 a, an outer protectivecover 67 b, and an inner protective cover 67 c.

The outer protective cover 67 b is a hollow cylinder formed of metal.The outer protective cover 67 b accommodates the inner protective cover67 c so as to cover it. The outer protective cover 67 b has a pluralityof inflow holes 67 b 1 formed in its peripheral wall. The inflow holes67 b 1 are through holes for allowing the exhaust gas EX (the exhaustgas which is present outside the outer protective cover 67 b) flowingthrough the exhaust passage to flow into the space inside the outerprotective cover 67 b. Further, the outer protective cover 67 b has anoutflow hole 67 b 2 formed in its bottom wall so as to allow the exhaustgas to flow from the space inside the outer protective cover 67 b to theoutside (exhaust passage).

The inner protective cover 67 c formed of metal is a hollow cylinderwhose diameter is smaller than that of the outer protective cover 67 b.The inner protective cover 67 c accommodates an air-fuel ratio detectingsection 67 a so as to cover it. The inner protective cover 67 c has aplurality of inflow holes 67 c 1 in its peripheral wall. The inflowholes 67 c 1 are through holes for allowing the exhaust gas, which hasflowed into the “space between the outer protective cover 67 b and theinner protective cover 67 c” through the inflow holes 67 b 1 of theouter protective cover 67 b, to flow into the space inside the innerprotective cover 67 c. In addition, the inner protective cover 67 c hasan outflow hole 67 c 2 formed in its bottom wall so as to allow theexhaust gas to flow from the space inside the inner protective cover 67c to the outside.

As shown in (A) to (C) of FIG. 2, the air-fuel ratio detecting section67 a includes a solid electrolyte layer 671, an exhaust-gas-sideelectrode layer 672, an atmosphere-side electrode layer 673, a diffusionresistance layer 674, a first partition section 675, a catalytic section676, a second partition section 677, and a heater 678.

The solid electrolyte layer 671 is formed of an oxygen-ion-conductivesintered oxide. In this embodiment, the solid electrolyte layer 671 is a“stabilized zirconia element” which is a solid solution of ZrO₂(zirconia) and CaO (stabilizer). The solid electrolyte layer 671exhibits an “oxygen cell property” and an “oxygen pump property,” whichare well known, when its temperature is equal to or higher than anactivation temperature thereof.

The exhaust-gas-side electrode layer 672 is formed of a noble metalhaving a high catalytic activity, such as platinum (Pt). Theexhaust-gas-side electrode layer 672 is formed on one of surfaces of thesolid electrolyte layer 671. The exhaust-gas-side electrode layer 672 isformed through chemical plating, etc. so as to exhibit adequate degreeof permeability (that is, it is formed into a porous layer).

The atmosphere-side electrode layer 673 is formed of a noble metalhaving a high catalytic activity, such as platinum (Pt). Theatmosphere-side electrode layer 673 is formed on the other one ofsurfaces of the solid electrolyte layer 671 in such a manner it facesthe exhaust-gas-side electrode layer 672 across the solid electrolytelayer 671. The atmosphere-side electrode layer 673 is formed throughchemical plating, etc. so as to exhibit adequate permeability (that is,it is formed into a porous layer).

The diffusion resistance layer (diffusion-controlling layer) 674 isformed of a porous ceramic material (heat-resistant inorganic material).The diffusion resistance layer 674 is formed through, for example,plasma spraying in such a manner that it covers the outer surface of theexhaust-gas-side electrode layer 672.

The first partition section 675 is formed of dense and gas-nonpermeablealumina ceramic. The first partition section 675 is formed so as tocover the diffusion resistance layer 674 except a corner (a part) of thediffusion resistance layer 674. That is, the first partition section 675has pass-through portions to expose parts of the diffusion resistancelayer 674 to the outside.

The catalytic section 676 is formed in the pass-through portions toclose the through hole. Similarly to the upstream catalyst 53, thecatalytic section 676 includes the catalytic substance whichfacilitates/accelerates the oxidation-reduction reaction and a substancefor storing oxygen which exerts the oxygen storage function. Thecatalytic section 676 is porous. Accordingly, as shown by a whitepainted arrow in (B) and (C) of FIG. 2, the exhaust gas (the abovedescribed exhaust gas which has flowed into the inside of the innerprotective cover 67 c) reaches the diffusion resistance layer 674through the catalytic section 676, and then further reaches theexhaust-gas-side electrode layer 672 through the diffusion resistancelayer 674.

The second partition section 677 is formed of dense and gas-nonpermeablealumina ceramic. The second partition section 677 is configured so as toform an “atmosphere chamber 67A” which is a space that accommodates theatmosphere-side electrode layer 673. Air is introduced into theatmosphere chamber 67A.

A power supply 679 is connected to the upstream air-fuel ratio sensor67. The power supply 679 applies a voltage V (=Vp) in such a manner thatthe atmosphere-side electrode layer 673 is held at a high potential andthe exhaust-gas-side electrode layer 672 is held at a low potential.

The heater 678 is buried in the second partition section 677. The heater678 produces heat when energized by the electric controller 70 so as toheat up the solid electrolyte layer 671, the exhaust-gas-side electrodelayer 672, and the atmosphere-side electrode layer 673 to adjusttemperatures of those. Hereinafter, “the solid electrolyte layer 671,the exhaust-gas-side electrode layer 672, and the atmosphere-sideelectrode layer 673” that are heated up by the heater 678 may bereferred to as “a sensor element section, or an air-fuel ratio sensorelement.” Accordingly, the heater 678 is configured so as to control the“air-fuel ratio sensor element temperature” which is the temperature ofthe sensor element section. An amount of energy supplied to the heater(and thus, the amount of heat generation) is adjusted to become greateras a duty signal (hereinafter, also referred to as a “heater duty Duty”)generated by the electric controller 70 becomes greater. When the heaterduty Duty is 100%, the amount of heat generation of the heater 678becomes maximum. When the heater duty Duty is 0%, energizing the heateris stopped, and accordingly, the heater does not produce any heat.

The admittance Y of the solid electrolyte layer 671 varies depending onthe air-fuel ratio sensor element temperature. In other words, theair-fuel ratio sensor element temperature can be estimated based on theadmittance Y. Generally, the admittance Y becomes larger as the air-fuelratio sensor element temperature becomes higher. The electric controller70 applies the “applied voltage generated by an electric power supply679” superimposed periodically with a “voltage having a rectangularwaveform, a sine waveform, or the like” between the exhaust-gas-sideelectrode layer 672 and the atmosphere-side electrode layer 673, andobtains the actual admittance Yact of the air-fuel ratio sensor 67 basedon the current flowing through the solid electrolyte layer 671.

As shown in (B) of FIG. 2, when the air-fuel ratio of the exhaust gas isleaner than the stoichiometric air-fuel ratio, the thus configuredupstream air-fuel ratio sensor 67 ionizes oxygen which has reached theexhaust-gas-side electrode layer 672 through the diffusion resistancelayer 674, and makes the ionized oxygen reach the atmosphere-sideelectrode layer 673. As a result, an electrical current I flows from apositive electrode of the electric power supply 679 to a negativeelectrode of the electric power supply 679. As shown in FIG. 3, themagnitude of the electrical current I becomes a constant value which isproportional to a concentration of oxygen arriving at theexhaust-gas-side electrode layer 672 (or a partial pressure, theair-fuel ratio of the exhaust gas), when the electric voltage V is setat a predetermined value Vp or higher. The upstream air-fuel ratiosensor 67 outputs a value into which this electrical current (i.e., thelimiting current Ip) is converted, as its output value Vabyfs.

To the contrary, as shown in (C) of FIG. 2, when the air-fuel ratio ofthe exhaust gas is richer than the stoichiometric air-fuel ratio, theupstream air-fuel ratio sensor 67 ionizes oxygen which is present in theatmosphere chamber 67A and makes the ionized oxygen reach theexhaust-gas-side electrode layer 672 so as to oxide the unburnedcombustibles (HC, CO, and H₂, etc.) reaching the exhaust-gas-sideelectrode layer 672 through the diffusion resistance layer 674. As aresult, an electrical current I flows from the negative electrode of theelectric power supply 679 to the positive electrode of the electricpower supply 679. As shown in FIG. 3, the magnitude of the electricalcurrent I also becomes a constant value which is proportional to aconcentration of the unburned combustibles arriving at theexhaust-gas-side electrode layer 672 (i.e., the air-fuel ratio of theexhaust gas), when the electric voltage V is set at the predeterminedvalue Vp or higher. The upstream air-fuel ratio sensor 67 outputs avalue into which the electrical current (i.e., the limiting current Ip)is converted, as its output value Vabyfs.

That is, the air-fuel detecting section 67 a, as shown in FIG. 4,outputs, as the “air-fuel ratio sensor output”, the output value Vabyfsbeing in accordance with the air-fuel ratio (an upstream air-fuel ratioabyfs, a detected air-fuel ratio abyfs) of the gas, which flows at theposition at which the upstream air-fuel ratio sensor 67 is disposed andreaches the air-fuel detecting section 67 a through the inflow holes 67b 1 of the outer protective cover 67 b and the inflow holes 67 c 1 ofthe inner protective cover 67 c. The output value Vabyfs becomes largeras the air-fuel ratio of the gas reaching the air-fuel ratio detectingsection 67 a becomes larger (leaner). That is, the output value Vabyfsis substantially proportional to the air-fuel ratio of the exhaust gasreaching the air-fuel ratio detecting section 67 a. It should be notedthat the output value Vabyfs becomes equal to a stoichiometric air-fuelratio corresponding value Vstoich, when the detected air-fuel ratioabyfs is equal to the stoichiometric air-fuel ratio.

The electric controller 70 stores an air-fuel ratio conversion table(map) Mapabyfs shown in FIG. 4, and detects an actual upstream air-fuelratio abyfs (that is, obtains the detected air-fuel ratio abyfs) byapplying the output value Vabyfs of the air-fuel ratio sensor 67 to theair-fuel ratio conversion table Mapabyfs.

Meanwhile, the upstream air-fuel ratio sensor 67 is disposed, in eitherthe exhaust manifold 51 or the exhaust pipe 52, at the position betweenthe exhaust merging portion HK of the exhaust manifold 51 and theupstream catalyst 53 in such a manner that the outer protective cover 67b is exposed.

More specifically, as shown in FIGS. 8 and 9, the air-fuel ratio sensor67 is disposed in such a manner that the bottom walls of the protectivecovers (67 b and 67 c) are parallel to the flow of the exhaust gas EXand the central axis CC of the protective covers (67 b and 67 c) isperpendicular to the flow of the exhaust gas EX. This allows the exhaustgas EX, which has reached the inflow holes 67 b 1 of the outerprotective cover 67 b, to be sucked into the space inside the outerprotective cover 67 b and into the space inside the inner protectivecover 67 c, owing to the flow of the exhaust gas EX in the exhaustpassage, which flows in the vicinity of the outflow hole 67 b 2 of theouter protective cover 67 b.

Thus, as indicated by the arrow Ar1 shown in FIGS. 8 and 9, the exhaustgas EX flowing through the exhaust passage flows into the space betweenthe outer protective cover 67 b and the inner protective cover 67 cthrough the inflow holes 67 b 1 of the outer protective cover 67 b.Subsequently, as indicated by the arrow Ar2, the exhaust gas flows intothe “the space inside the inner protective cover 67 c” through the“inflow holes 67 c 1 of the inner protective cover 67 c,” and thenreaches the air-fuel ratio detection element 67 a. Thereafter, asindicated by the arrow Ar3, the exhaust gas flows out to the exhaustpassage through the “outflow hole 67 c 2 of the inner protective cover67 c and the outflow hole 67 b 2 of the outer protective cover 67 b.”

Accordingly, the flow rate of the exhaust gas within “the outerprotective cover 67 b and the inner protective cover 67 c” changes inaccordance with the flow rate of the exhaust gas EX flowing near theoutflow hole 67 b 2 of the outer protective cover 67 b (i.e., the intakeair flow rate Ga representing the intake air amount per unit time). Inother words, a time duration from a “point in time at which an exhaustgas having a specific air-fuel ratio (first exhaust gas) reaches theinflow holes 67 b 1” to a “point in time at which the first exhaust gasreaches the air-fuel ratio detecting section 67 a” depends on the intakeair-flow rate Ga, but does not depend on the engine rotational speed NE.Accordingly, the output responsiveness (responsiveness) of the air-fuelratio sensor 67 for (with respect to) the “air-fuel ratio of the exhaustgas flowing through the exhaust passage” becomes better as the flow rate(speed of flow) of the exhaust gas flowing in the vicinity of the outerprotective cover 67 b is higher. This can be true even in a case inwhich the upstream air-fuel ratio sensor 67 has the inner protectivecover 67 c only.

Referring back to FIG. 7 again, the downstream air-fuel ratio sensor 68is disposed in the exhaust pipe 52, and at a position downstream of anupstream catalyst 53 and upstream of the downstream catalyst (i.e., inthe exhaust passage between the upstream catalyst 53 and the downstreamcatalyst). The downstream air-fuel ratio sensor 68 is a well-knownelectro-motive-force-type oxygen concentration sensor (well-knownconcentration-cell-type oxygen concentration sensor using stabilizedzirconia). The downstream air-fuel ratio sensor 68 is designed togenerate an output value Voxs corresponding to the air-fuel ratio of agas to be detected, the gas flowing through a portion of the exhaustpassage at which the downstream air-fuel ratio sensor 68 is disposed(that is, the air-fuel ratio of the gas which flows out from theupstream catalyst 53 and flows into the downstream catalyst; namely, thetime average (temporal mean value) of the air-fuel ratio of the mixturesupplied to the engine).

As shown in FIG. 10, this output value Voxs becomes a “maximum outputvalue max (e.g., about 0.9 V)” when the air-fuel ratio of the exhaustgas to be detected is richer than the stoichiometric air-fuel ratio,becomes a “minimum output value min (e.g., about 0.1 V) when theair-fuel ratio of the exhaust gas to be detected is leaner than thestoichiometric air-fuel ratio, and becomes a voltage Vst (midpointvoltage Vst, e.g., about 0.5 V) which is approximately the midpointvalue between the maximum output value max and the minimum output valuemin when the air-fuel ratio of the exhaust gas to be detected is thestoichiometric air-fuel ratio. Further, this voltage Vox changessuddenly from the maximum output value max to the minimum output valuemin when the air-fuel ratio of the exhaust gas to be detected changesfrom the air-fuel ratio richer than the stoichiometric air-fuel ratio tothe air-fuel ratio leaner than the stoichiometric air-fuel ratio, andchanges suddenly from the minimum output value min to the maximum outputvalue max when the air-fuel ratio of the exhaust gas to be detectedchanges from the air-fuel ratio leaner than the stoichiometric air-fuelratio to the air-fuel ratio richer than the stoichiometric air-fuelratio.

The accelerator opening sensor 69 shown in FIG. 7 is designed to outputa signal which indicates the operation amount Accp of the acceleratorpedal 81 operated by the driver (accelerator pedal operation amountAccp). The accelerator pedal operation amount Accp increases as theopening of the accelerator pedal 81 (accelerator pedal operation amount)increases.

The electric controller 70 is a well-known microcomputer which includesa CPU 71; a ROM 72 in which programs executed by the CPU 71, tables(maps and/or functions), constants, etc. are stored in advance; a RAM 73in which the CPU 71 temporarily stores data as needed; a backup RAM 74;and an interface 75 which includes an AD converter, etc. Thesecomponents are mutually connected via a bus.

The backup RAM 74 is supplied with an electric power from a batterymounted on a vehicle on which the engine 10 is mounted, regardless of aposition (off-position, start position, on-position, and so on) of anunillustrated ignition key switch of the vehicle. While the electricpower is supplied to the backup RAM 74, data is stored in (written into)the backup RAM 74 according to an instruction of the CPU 71, and thebackup RAM 74 holds (retains, stores) the data in such a manner that thedata can be read out. When the battery is taken out from the vehicle,and thus, when the backup RAM 74 is not supplied with the electricpower, the backup RAM 74 can not hold the data. Accordingly, the CPU 71initializes the data (sets the data to default values) to be stored inthe backup RAM 74 when the electric power starts to be supplied to thebackup RAM 74 again.

The interface 75 is connected to sensors 61 to 69 so as to send signalsfrom these sensors to the CPU 71. In addition, the interface 75 isdesigned to send drive signals (instruction signals) to the actuator 33a of the variable intake timing control apparatus 33, the actuator 36 aof a variable exhaust timing control apparatus 36, each of the igniters38 of the cylinders, the fuel injection valves 39 each of which isprovided for each of the cylinders, the throttle valve actuator 44 a,the heater 678 of the air-fuel ratio sensor 67, etc., in response toinstructions from the CPU 71.

The electric controller 70 is designed to send an instruction signal tothe throttle valve actuator 44 a so that the throttle valve opening TAincreases as the obtained accelerator pedal operation amount Accpincreases. That is, the electric controller 70 has throttle valve drivemeans for changing the opening of the “throttle valve 44 disposed in theintake passage of the engine 10” in accordance with the accelerationoperation amount (accelerator pedal operation amount Accp) of the engine10 which is changed by the driver.

(Outline of the Inter-Cylinder Air-Fuel Ratio Imbalance Determination)

Next, there will be described the outline of method for the“inter-cylinder air-fuel ratio imbalance determination” which isadopted/used by the first determination apparatus. The inter-cylinderair-fuel ratio imbalance determination is to determine whether or notnon-uniformity of the air-fuel ratio among the cylinders exceeds a valuerequiring some warning due to the change of the property/characteristicof the fuel injection valve 39, etc. In other words, the firstdetermination apparatus determines that the inter-cylinder air-fuelratio imbalance state has occurred when the magnitude of the differencein air-fuel ratio (cylinder-by-cylinder air-fuel ratio difference)between the imbalanced cylinder and the balanced cylinder is equal to orlarger than a “degree which is not permissible in terms of theemission”.

The first determination apparatus obtains, in order to perform theinter-cylinder air-fuel ratio imbalance determination, the change amountper unit time (constant sampling time ts) of the air-fuel ratiorepresented by the output value Vabyfs of the air-fuel ratio sensor 67(i.e., the detected air-fuel ratio abyfs obtained by applying the outputvalue Vabyfs to the air-fuel ratio conversion table Mapabyfs shown inFIG. 4). The “change amount of the detected air-fuel ratio abyfs perunit time” can be said as (to be) a temporal (or time) differentialvalue d(abyfs)/dt of the detected air-fuel ratio abyfs, if the unit timeis very short, e.g., about 4 ms. Accordingly, the “change amount of thedetected air-fuel ratio abyfs per unit time” will also simply bereferred to as a “detected air-fuel ratio change rate ΔAF.”

Exhaust gases from the cylinders reach the air-fuel ratio sensor 67 inthe order of ignition (namely, in the order of exhaust). If theinter-cylinder air-fuel ratio imbalance state has not being occurring,the air-fuel ratios of the exhaust gases which are discharged from thecylinders and reach the air-fuel ratio sensor 67 are almost the same toeach other. Accordingly, when the inter-cylinder air-fuel ratioimbalance state has not being occurring, the detected air-fuel ratioabyfs changes, for example, as indicated by a broken line C1 in (B) ofFIG. 5. That is, when the inter-cylinder air-fuel ratio imbalance statehas not being occurring, the waveforms of the output value Vabyfs of theair-fuel ratio sensor 67 are nearly flat. Thus, as shown by a brokenline C3 in (C) of FIG. 5, when the inter-cylinder air-fuel ratioimbalance state has not being occurring, an absolute value of thedetected air-fuel ratio change rate ΔAF is small.

Meanwhile, when the property of the “injection valve 39 injecting fuelto a specific cylinder (e.g., the first cylinder)” becomes a propertythat it injects fuel in an “amount greater than the instructed fuelinjection amount”, and thus, the inter-cylinder air-fuel ratio imbalancestate has occurred, an air-fuel ratio of an exhaust gas of the specificcylinder (air-fuel ratio of the imbalanced cylinder) is greatlydifferent from air-fuel ratios of exhaust gases of cylinders other thanthe specific cylinder (air-fuel ratio of the balanced cylinder).

Accordingly, the detected air-fuel ratio abyfs when the inter-cylinderair-fuel ratio imbalance state is occurring changes/fluctuates greatlyat an interval of the unit combustion cycle, as indicated by a solidline C2 in (B) of FIG. 12. Therefore, as shown by a solid line C4 in (C)of FIG. 5, when the inter-cylinder air-fuel ratio imbalance state isoccurring, the absolute value of the detected air-fuel ratio change rateΔAF becomes large. It should be noted that, in a case where the engineis an in-line four-cylinder four-cycle type, the unit combustion cycleperiod is a period for which a crank angle of 720° passes/elapses, thatis, a period for which a crank angle passes, the crank angle beingrequired for the engine to complete one combustion stroke in every andall of the cylinders that are the first to fourth cylinders, whichdischarge exhaust gases reaching the single air-fuel ratio sensor 67.

Furthermore, the absolute value |ΔAF| of the detected air-fuel ratiochange rate ΔAF fluctuates more greatly as the air-fuel ratio of theimbalanced cylinder deviates more greatly from the air-fuel ratio of thebalanced cylinder. For example, if the detected air-fuel ratio abyfschanges as indicated by the solid line C2 in (B) of FIG. 5 when amagnitude of a difference between the air-fuel ratio of the imbalancedcylinder and the air-fuel ratio of the balanced cylinder is equal to afirst value, the detected air-fuel ratio abyfs changes as indicated byan alternate long and short dash line C2 a in (B) of FIG. 5 when themagnitude of the difference between the air-fuel ratio of the imbalancedcylinder and the air-fuel ratio of the balanced cylinder is equal to a“second value larger than the first value.” Accordingly, the absolutevalue of the detected air-fuel ratio change rate ΔAF becomes larger asthe air-fuel ratio of the imbalanced cylinder deviates more greatly fromthe air-fuel ratio of the balanced cylinder.

In view of the above, the first determination apparatus obtains, as abase indicating amount, the detected air-fuel ratio change rate ΔAFevery time the sampling time is elapses in a single unit combustioncycle period during/over a period (parameter obtaining period) in whicha predetermined parameter obtaining condition is satisfied. The firstdetermination apparatus obtains a mean value (an average value) of theabsolute values |ΔAF| of a plurality of the detected air-fuel ratiochange rates ΔAF obtained in the single unit combustion cycle period.Further, the first determination apparatus obtains a mean (average)value of the “mean values (average values) of the absolute values|ΔAF|”, each has been obtained for each of a plurality of the combustioncycle periods, and adopts the obtained value as the air-fuel ratiofluctuation indicating amount AFD and as the imbalance determinationparameter X. It should be noted that the imbalance determinationparameter X is not limited to the above-described value, but may beobtained according to various methods described later.

Meanwhile, the first determination apparatus controls the air-fuel ratiosensor element temperature using the amount of heat generation by theheater 678. The first determination apparatus controls the air-fuelratio sensor element temperature to be a first temperature (usualtemperature) t1 in a period other than the parameter obtaining period(i.e., a period in which the detected air-fuel ratio change rate ΔAFserving as a base for the imbalance determination parameter is not beingobtained). When the air-fuel ratio sensor element temperature is thefirst temperature, the air-fuel ratio sensor 67 is active, and theoutput value Vabyfs of the air-fuel ratio sensor 67 represents/indicatesthe air-fuel ratio of the exhaust gas. However, the responsiveness ofthe air-fuel ratio sensor 67 is relatively low, and therefore, theoutput value can not follow the quick change of the air-fuel ratio ofthe exhaust gas sufficiently.

In view of the above, the first determination apparatus controls theair-fuel ratio sensor element temperature to be a “second temperature(elevated temperature) t2 higher than the first temperature” in theparameter obtaining period (i.e., a period in which the detectedair-fuel ratio change rate ΔAF is being obtained). Consequently, theresponsiveness of the air-fuel ratio sensor 67 when the detectedair-fuel ratio change rate ΔAF is obtained is (made) higher than theresponsiveness of the air-fuel ratio sensor 67 when the detectedair-fuel ratio change rate ΔAF is not obtained.

As a result, the first determination apparatus can obtain the imbalancedetermination parameter X while the responsiveness of the air-fuel ratiosensor 67 is made higher. The imbalance determination parameter Xobtained by the first determination apparatus therefore accuratelyrepresents the “degree of the inter-cylinder air-fuel ratio imbalancestate (the cylinder-by-cylinder air-fuel ratio difference).”

After the first determination apparatus obtains the imbalancedetermination parameter X, it compares the imbalance determinationparameter X with an imbalance determination threshold Xth. The firstdetermination apparatus determines that the inter-cylinderair-fuel-ratio imbalance state has occurred when the imbalancedetermination parameter X is larger than the imbalance determinationthreshold Xth. In contrast, the first determination apparatus determinesthat the inter-cylinder air-fuel-ratio imbalance state has not occurredwhen the imbalance determination parameter X is smaller than theimbalance determination threshold Xth. This is the outline of the methodof inter-cylinder air-fuel-ratio imbalance determination employed by thefirst determination apparatus.

(Actual Operation)

<Fuel Injection Amount Control>

The CPU 71 of the first determination apparatus is designed torepeatedly execute a “routine for calculating the instructed fuelinjection amount Fi and for instructing a fuel injection” shown in FIG.12 for an arbitrary cylinder (hereinafter also referred to as a “fuelinjection cylinder”) each time the crank angle of that cylinder reachesa predetermined crank angle before its intake top dead center (e.g.,BTDC 90° CA). Accordingly, when the predetermined timing comes, the CPU71 starts processing from step 1200, and determines whether or not afuel cut condition (hereinafter, expresses as “FC condition”) issatisfied at step 1210.

It is assumed here that the FC condition is not satisfied. In this case,the CPU 71 makes a “No” determination at step 1210 to executes processesfrom step 1220 to step 1250. Thereafter, the CPU 71 proceeds to step1295 to end the present routine tentatively.

Step 1210: The CPU 71 obtains an “in-cylinder intake air amount Mc(k)”,namely, the “amount of air taken into the fuel injection cylinder”,based on the “intake air flow rate Ga measured using the air flow meter61, the engine rotational speed NE obtained based on the signal from thecrank position sensor 64, and a lookup table MapMc.” The in-cylinderintake air amount Mc(k) is stored with information specifying the intakestroke in the RAM. The in-cylinder intake air amount Mc(k) may becomputed from a well-known air model (a model established in conformitywith a physical law simulating the behavior of air in the intakepassage).

Step 1230: The CPU 71 obtains a basic fuel injection amount Fbasethrough dividing the in-cylinder intake air amount Mc(k) by a targetair-fuel ratio abyfr. The target air-fuel ratio abyfr (upstream-sidetarget air-fuel ratio abyfr) is set to (at) the stoichiometric air-fuelratio (e.g., 14.6) except for specific cases, such as a case after thestart or a case in which the load is high. Accordingly, the basic fuelinjection amount Fbase is a feedforward amount of the fuel injectionamount which is required for realizing/achieving the target air-fuelratio abyfr which is equal to the stoichiometric air-fuel ratio. Thestep 1230 constitutes feedforward control means (air-fuel ratio controlmeans) for having the air-fuel ratio of the mixture supplied to theengine coincide with the target air-fuel ratio abyfr.

Step 1240: The CPU 71 corrects the basic fuel injection amount Fbasebased on a main feedback amount DFi. More specifically, the CPU 71computes the instructed fuel injection amount (final fuel injectionamount) Fi by adding the main feedback amount DFi to the basic fuelinjection amount Fbase. The main feedback amount DFi is an air-fuelratio feedback amount to have the air-fuel ratio of the engine coincidewith the target air-fuel ratio abyfr. A way of calculating of the mainfeedback amount DFi will be described later.

Step 1250: The CPU 71 sends the injection instruction signal to the fuelinjection valve 39 provided for the fuel injection cylinder, so thatfuel of the instructed injection amount Fi is injected from that fuelinjection valve 39.

Consequently, the fuel of an amount required to have the air-fuel ratioof the engine coincide with the target air-fuel ratio abyfr (in mostcases, the stoichiometric air-fuel ratio) is injected from the fuelinjection valve 39 of the fuel injection cylinder. That is, steps from1220 to 1250 constitute instructed fuel injection amount control meansfor controlling the instructed fuel injection amount Fi in such a mannerthat an “air-fuel ratio of the mixture supplied to the combustionchambers 25 of two or more of the cylinders (in the present example, allof the cylinder) which discharge the exhaust gases reaching the air-fuelratio sensor 67.”

Meanwhile, if the FC condition is satisfied when the CPU 71 executes theprocess of step 1210, the CPU 71 makes a “Yes” determination at step1210 to directly proceed to step 1295 so as to end the present routinetentatively. In this case, fuel injection is not carried out by theprocess of step 1250, and the fuel cut control (fuel supply stopcontrol) is therefore performed.

<Computation of the Main Feedback Amount>

The CPU 71 repeatedly executes a “main feedback amount computationroutine” shown by a flowchart of FIG. 13 every time a predetermined timeelapses. Accordingly, when the predetermined timing comes, the CPU 71starts processing from step 1300, and proceeds to step 1305 to determinewhether or not a “main feedback control condition (upstream-sideair-fuel ratio feedback control condition)” is satisfied.

The main feedback control condition is satisfied when all of thefollowing conditions are satisfied:

(A1) The air-fuel ratio sensor 67 has been activated.(A2) An engine load KL is equal to or smaller than a threshold KLth.(A3) The fuel cut control is not being performed.

It should be noted that, in the present embodiment, the load KL is aloading rate obtained in accordance with a formula (1) given below. Anaccelerator pedal operation amount Accp may be used in place of the loadfactor KL. In the formula (1), Mc is the in-cylinder intake air amount,ρ is the density of air (unit: g/l), L is the displacement of the engine10 (unit: I), and “4” is the number of the cylinders of the engine 10.

KL=(Mc/(ρ·L/4))·100%  (1)

A description will be continued on the assumption that the main feedbackcontrol condition is satisfied. In this case, the CPU 71 makes a “Yes”determination at step 1305 to execute processes from steps 1310 to 1340described below one after another, and then proceeds to step 1395 to endthe present routine tentatively.

Step 1310: The CPU 71 obtains an output value Vabyfc for a feedbackcontrol, according to a formula (2) described below. In the formula (2),Vabyfs is the output value of the air-fuel ratio sensor 67, Vafsfb is asub feedback amount calculated based on the output value Voxs of thedownstream air-fuel ratio sensor 68. The way by which the sub feedbackamount Vafsfb is calculated is well known. For example, the sub feedbackamount Vafsfb is decreased when the output value Voxs of the downstreamair-fuel ratio sensor 68 is a value indicating an air-fuel ratio richerthan the stoichiometric air-fuel ratio corresponding to the value Vst,and is increased when the output value Voxs of the downstream air-fuelratio sensor 68 is a value indicating an air-fuel ratio leaner than thestoichiometric air-fuel ratio corresponding to the value Vst. Note thatthe first determination apparatus may set the sub feedback amount Vafsfbto (at) “0”, so that it may not perform the sub feedback control.

Vabyfc=Vabyfs+Vafsfb  (2)

Step 1315: The CPU 71 obtains an air-fuel ratio abyfsc for a feedbackcontrol by applying the output value Vabyfc for a feedback control tothe table Mapabyfs shown in FIG. 4, as shown by a formula (3) describedbelow.

abyfsc=Mapabyfs(Vabyfc)  (3)

Step 1320: According to a formula (4) described below, the CPU 71obtains a “in-cylinder fuel supply amount Fc(k−N)” which is an “amountof the fuel actually supplied to the combustion chamber 25 for a cycleat a timing N cycles before the present time.” That is, the CPU 71obtains the “in-cylinder fuel supply amount Fc(k−N)” through dividingthe “in-cylinder intake air amount Mc(k−N) which is the in-cylinderintake air amount for the cycle the N cycles (i.e., N·720° crank angle)before the present time” by the “air-fuel ratio abyfsc for a feedbackcontrol.”

Fc(k−N)=Mc(k−N)/abyfsc  (4)

The reason why the in-cylinder intake air amount Mc(k−N) for the cycle Ncycles before the present time is divided by the air-fuel ratio abyfscfor a feedback control in order to obtain the in-cylinder fuel supplyamount Fc(k−N) is because the “exhaust gas generated by the combustionof the mixture in the combustion chamber 25” requires time“corresponding to the N cycles” to reach the air-fuel ratio sensor 67.

Step 1325: The CPU 71 obtains a “target in-cylinder fuel supply amountFcr(k−N)” which is a “fuel amount which was supposed to be supplied tothe combustion chamber 25 for the cycle the N cycles before the presenttime”, according to a formula (5) described below. That is, the CPU 71obtains the target in-cylinder fuel supply amount Fcr(k−N) throughdividing the in-cylinder intake air amount Mc(k−N) for the cycle the Ncycles before the present time by the target air-fuel ratio abyfr.

Fcr(k−N)=Mc(k−N)/abyfr  (5)

Step 1330: The CPU 71 obtains an “error DFc of the in-cylinder fuelsupply amount”, according to a formula (6) described below. That is, theCPU 71 obtains the error DFc of the in-cylinder fuel supply amount bysubtracting the in-cylinder fuel supply amount Fc(k−N) from the targetin-cylinder fuel supply amount Fcr(k−N). The error DFc of thein-cylinder fuel supply amount represents excess and deficiency of thefuel supplied to the cylinder the N cycle before the present time.

DFc=Fcr(k−N)−Fc(k−N)  (6)

Step 1335: The CPU 71 obtains the main feedback amount DFi, according toa formula (7) described below. In the formula (7) below, Gp is apredetermined proportion gain, and Gi is a predetermined integrationgain. Further, a “value SDFc” in the formula (7) is an “integrated valueof the error DFc of the in-cylinder fuel supply amount”. That is, theCPU 71 calculates the “main feedback amount DFi” based on aproportional-integral control to have the air-fuel ratio abyfsc for afeedback control become equal to the target air-fuel ratio abyfr.

DFi=Gp·DFc+Gi·SDFc  (7)

Step 1340: The CPU 71 obtains a new integrated value SDFc of the errorof the in-cylinder fuel supply amount by adding the error DFc of thein-cylinder fuel supply amount obtained at the step 1330 to the currentintegrated value SDFc of the error DFc of the in-cylinder fuel supplyamount.

As described above, the main feedback amount DFi is obtained based onthe proportional-integral control. The main feedback amount DFi isreflected in (onto) the final fuel injection amount Fi by the process ofthe step 1240 shown in FIG. 12.

In contrast, when the determination is made at step 1305, and if themain feedback condition is not satisfied, the CPU 71 makes a “No”determination at step 1305 to proceed to step 1345, at which the CPU 71sets the value of the main feedback amount DFi to (at) “0”.Subsequently, the CPU 71 stores “0” into the integrated value SDFc ofthe error of the in-cylinder fuel supply amount at step 1350.Thereafter, the CPU 71 proceeds to step 1395 to end the present routinetentatively. As described above, when the main feedback condition is notsatisfied, the main feedback amount DFi is set to (at) “0”. Accordingly,the correction for the basic fuel injection amount Fbase with the mainfeedback amount DFi is not performed.

<Inter-Cylinder Air-Fuel Ratio Imbalance Determination>

Next, there will be described processing for performing “inter-cylinderair-fuel ratio imbalance determination.” The CPU 71 is designed toexecute an “inter-cylinder air-fuel ratio imbalance determinationroutine” shown in the flowchart of FIG. 14 every time 4 ms(predetermined, fixed sampling interval ts) elapses.

Therefore, when a predetermined timing comes, the CPU 71 startsprocessing from step 1400, and then proceeds to step 1405 to determinewhether or not a value of a parameter obtaining permission flag Xkyokais “1.”

The value of the parameter obtaining permission flag Xkyoka is set to(at) “1”, when a parameter obtaining condition (imbalance determinationparameter obtaining permissible condition) described later is satisfiedat a point in time at which the absolute crank angle CA reaches 0° crankangle, and is set to (at) “0” immediately after a point in time at whichthe parameter obtaining condition becomes unsatisfied.

The parameter obtaining condition is satisfied when all of conditionsdescribed below (conditions C1 to C6) are satisfied. Accordingly, theparameter obtaining condition is unsatisfied when at least one of theconditions described below (conditions C1 to C6) is unsatisfied. Itshould be noted that the conditions constituting the parameter obtainingcondition are not limited to those conditions C1 to C6 described below.

(Condition 1) A final result as to the inter-cylinder air-fuel-ratioimbalance determination has not been obtained yet after the currentstart of the engine 10. The condition C1 is also referred to as animbalance determination execution request condition. The condition C1may be replaced by a condition satisfied when “an integrated value of anoperation time of the engine or an integrated value of the intake airflow rate Ga is equal to or larger than a predetermined value.”

(Condition 2) The intake air flow rate Ga measured by the air-flow meter61 is within a predetermined range. That is, the intake air flow rate Gais equal to or larger than a low-side intake air flow rate thresholdGaLoth and is equal to or smaller than a high-side intake air flow ratethreshold GaHith.(Condition 3) The engine rotational speed NE is within a predeterminedrange. That is, the engine rotational speed NE is equal to or largerthan a low-side engine rotational speed NELoth and is equal to orsmaller than a high-side engine rotational speed NEHith.(Condition 4) The cooling water temperature THW is equal to or higherthan a threshold cooling water temperature THWth.(Condition 5) The main feedback control condition is satisfied.(Condition 6) The fuel cut control is not being performed.

It is assumed here that the value of the parameter obtaining permissionflag Xkyoka is equal to “1”. In this case, the CPU 71 makes a “Yes”determination at step 1405 to proceed to step 1410, at which the CPU 71sets a value of a sensor element temperature elevation request flagXtupreq to (at) “1.” The value of the sensor element temperatureelevation request flag Xtupreq is set to (at) “0” in an initial routine.The initial routine is a routine which is executed by the CPU 71 whenthe ignition key switch of the vehicle equipped with the engine 10 isturned from the OFF position to the ON position.

When the value of the sensor element temperature elevation request flagXtupreq is set to (at) “1”, the heater duty Duty representing the amountof energy supplied to the heater is increased by processing an “air-fuelratio sensor heater control routine” shown in FIG. 15 described later,the temperature (air-fuel ratio sensor element temperature) of theair-fuel ratio detecting section 67 a (especially, the sensor elementsection comprising the solid electrolyte layer 671, the exhaust-gas-sideelectrode layer 672, and the atmosphere-side electrode layer 673) iselevated/raised from the “first temperature (usual temperature) t1serving as the parameter-non-obtaining-period-element-temperature” tothe “second temperature (elevated temperature) t2 serving as theparameter-obtaining-period-element-temperature.” As a result, theresponsiveness of the air-fuel ratio sensor 67 becomes higher (refer toFIG. 6).

Subsequently, the CPU 71 proceeds to step 1415, at which the CPU 71determines whether or not a delay time (a predetermined time) Tdelaythhas elapsed since a point in time at which the value of the sensorelement temperature elevation request flag Xtupreq was changed from “0”to “1.” When the delay time Tdelayth has not elapsed since the point intime at which the value of the sensor element temperature elevationrequest flag Xtupreq was changed from “0” to “1”, the CPU 71 makes a“No” determination at step 1415 to directly proceed to step 1495 to endthe present routine tentatively.

In contrast, at a point in time at which the CPU 71 executes the processof step 1415, if the delay time Tdelayth has elapsed since the point intime at which the value of the sensor element temperature elevationrequest flag Xtupreq was changed from “0” to “1”, the CPU 71 proceedsfrom step 1415 to step 1420, at which the CPU 71 obtains the “outputvalue of the air-fuel ratio sensor 67 at that point in time” through anAD conversion. It should be noted that step 1415 may be omitted. In thiscase, the CPU 71 directly proceeds to step 1420 after step 1410.

Subsequently, the CPU proceeds to step 1425 to obtain a present/currentdetected air-fuel ratio abyfs by applying the output value Vabyfsobtained at step 1420 to the air-fuel ratio conversion table Mapabyfsshown in FIG. 4. It should be noted that the CPU 71 stores the detectedair-fuel ratio obtained when the present routine was previously executedas a previous detected air-fuel ratio abyfsold, before the process ofstep 1420. That is, the previous detected air-fuel ratio abyfsold is thedetected air-fuel ratio abyfs 4 ms (the sampling time ts) before thepresent time. An initial value of the previous detected air-fuel ratioabyfsold is set at a value corresponding to an AD-converted value of thestoichiometric air-fuel ratio in the above-described initial routine.

Subsequently, the CPU 71 proceeds to step 1430, at which the CPU 71,

(A) obtains the detected air-fuel ratio changing rate SAF,(B) renews/updates a cumulated value SAFD of an absolute value |ΔAF| ofthe detected air-fuel ratio changing rate ΔAF, and(C) renews/updates a cumulated number counter Cn showing how many timesthe absolute value |ΔAF| of the detected air-fuel ratio changing rateΔAF is accumulated (integrated) to the cumulated value SAFD.

Next will be described the ways in which these values are renewed morespecifically.

(A) Obtainment of the detected air-fuel ratio change rate ΔAF:

The detected air-fuel ratio change rate ΔAF (differential valued(abyfs)/dt) is a data (basic indicating amount) which is a base datafor the air-fuel ratio fluctuation indicating amount AFD as well as theimbalance determination parameter X. The CPU 71 obtains the detectedair-fuel ratio change rate ΔAF by subtracting the previous detectedair-fuel ratio abyfsold from the present detected air-fuel ratio abyfs.That is, when the present detected air-fuel ratio abyfs is expressed asabyfs(n) and the previous detected air-fuel ratio abyfs is expressed asabyfs(n−1), the CPU 71 obtains the “present detected air-fuel ratiochange rate ΔAF(n)” at step 1430, according to a formula (8) describedbelow.

ΔAF(n)=abyfs(n)−abyfs(n−1)  (8)

(B) Renewal of the integrated value SAFD of the absolute value |ΔAF| ofthe detected air-fuel ratio change rate ΔAF:

The CPU 71 obtains the present integrated value SAFD(n) according to aformula (9) described below. That is, the CPU 71 renews the integratedvalue SAFD by adding the absolute value |ΔAF(n)| of the present detectedair-fuel ratio change rate ΔAF(n) calculated as above to the previousintegrated value SAFD(n−1) at the point in time when the CPU 71 proceedsto step 1430.

SAFD(n)=SAFD(n−1)+|ΔAF(n)|  (9)

The reason why the “absolute value |ΔAF(n)| of the present detectedair-fuel ratio change rate” is added to the integrated value SAFD isthat the detected air-fuel ratio change rate ΔAF(n) can become both apositive value and a negative value, as understood from (B) and (C) inFIG. 5. It should be noted that the integrated value SAFD is set to (at)“0” in the initial routine described above.

(C) Renewal of the cumulated number counter Cn of the absolute value|ΔAF| of the detected air-fuel ratio change rate ΔAF added to theintegrated value SAFD:

The CPU 71 increments a value of the counter Cn by “1” according to aformula (10) described below. Cn(n) represents the counter Cn after therenewal, and Cn(n−1) represents the counter Cn before the renewal. Thevalue of the counter Cn is set to (at) “0” in the initial routinedescribed above, and is also set to (at) “0” at step 1475 describedlater. The value of the counter Cn therefore represents the number ofdata of the absolute value |ΔAF| of the detected air-fuel ratio changerate ΔAF which has been accumulated in the integrated value SAFD.

Cn(n)=Cn(n−1)+1  (10)

Subsequently, the CPU 71 proceeds to step 1435 to determine whether ornot the crank angle CA (the absolute crank angle CA) measured withreference to the top dead center of the compression stroke of thereference cylinder (in the present example, the first cylinder) reaches720° crank angle. When the absolute crank angle CA is less than 720°crank angle, the CPU 71 makes a “No” determination at step 1435 todirectly proceed to step 1495 at which the CPU 71 ends the presentroutine tentatively.

It should be noted that step 1435 is a step to define the smallest unitperiod for obtaining a mean value (or average) of the absolute values|ΔAF| of the detected air-fuel ratio change rates ΔAF. Here, the “720°crank angle which is the unit combustion cycle” corresponds to thesmallest unit period. The smallest unit period may obviously be shorterthan the 720° crank angle, however, may preferably be a time periodlonger than or equal to a period having an integral multiple of thesampling time ts. That is, it is preferable that the smallest unitperiod be set/determined in such a manner that a plurality of thedetected air-fuel ratio change rates ΔAF are obtained in the smallestunit period.

Meanwhile, if the absolute crank angle CA reaches 720° crank angle whenthe CPU 71 executes the process of step 1435, the CPU 71 makes a “Yes”determination at step 1435 to proceed to step 1440.

The CPU 71, at step 1440:

(D) calculates a mean value (average) AveΔAF of the absolute values|ΔAF| of the detected air-fuel ratio change rates ΔAF,(E) renews/updates an integrated value Save of the mean value AveΔAF,and(F) renews/updates a cumulated number counter Cs.

The ways in which these values are renewed will be next be describedmore specifically.

(D) Calculation of the mean value AveΔAF of the absolute values |ΔAF| ofthe detected air-fuel ratio change rates ΔAF:

The CPU 71 calculates the mean value AveΔAF (=SAFD/Cn) of the absolutevalues |ΔAF| of the detected air-fuel ratio change rates ΔAF by dividingthe integrated value SAFD by the value of the counter Cn, as shown in aformula (11) described below.

AveΔAF=SAFD/Cn  (11)

(E) Renewal of the integrated value Save of the mean value AveΔAF:

The CPU 71 obtains the present integrated value Save(n) according to aformula (12) described below. That is, the CPU 71 renews the integratedvalue Save by adding the present mean value AveΔAF obtained as describedabove to the previous integrated value Save(n−1) at the point in timewhen the CPU 71 proceeds to step 1440. The value of the integrated valueSave(n) is set to (at) “0” in the initial routine described above.

Save(n)=Save(n−1)+AveΔAF  (12)

(F) Renewal of the cumulated number counter Cs:

The CPU 71 increments a value of the counter Cs by “1” according to aformula (13) described below. Cs(n) represents the counter Cs after therenewal, and Cs(n−1) represents the counter Cs before the renewal. Thevalue of the counter Cs is set to (at) “0” in the initial routinedescribed above. The value of the counter Cs therefore represents thenumber of data of the mean value AveΔAF which has been accumulated inthe integrated value Save.

Cs(n)=Cs(n−1)+1  (13)

Subsequently, the CPU 71 proceeds to step 1445 to determine whether ornot the value of the counter Cs is larger than or equal to a thresholdvalue Csth. When the value of the counter Cs is smaller than thethreshold value Csth, the CPU 71 makes a “No” determination at step 1445to directly proceed to step 1495 at which the CPU 71 ends the presentroutine tentatively. It should be noted that the threshold value Csth isa natural number, and is preferably larger than or equal to 2.

Meanwhile, if the value of the counter Cs is larger than or equal to thethreshold value Csth when the CPU 71 executes the process of step 1445,the CPU 71 makes a “Yes” determination at step 1445 to execute processesof step 1450 and step 1455 one after another, and then proceeds to step1460.

Step 1450: The CPU 71 obtains the air-fuel ratio fluctuation indicatingamount AFD through dividing the integrated value Save by the value ofthe counter (=Csth) according to a formula (14) described below. Theair-fuel ratio fluctuation indicating amount AFD is a value obtained byaveraging the mean values of the absolute values |ΔAF| of the detectedair-fuel ratio change rates ΔAF, each of the mean values being obtainedfor each of the unit combustion cycle periods, over a plurality (Csth)of the unit combustion cycle periods.

AFD=Save/Csth  (14)

Step 1455: The CPU 71 obtains, as the imbalance determination parameterX, the air-fuel ratio fluctuation indicating amount AFD obtained at step1450.

Subsequently, the CPU 71 proceeds to step 1460 to determine whether ornot the imbalance determination parameter X is larger than an imbalancedetermination threshold Xth.

When the imbalance determination parameter X is larger than theimbalance determination threshold Xth, the CPU 71 makes a “Yes”determination at step 1460 to proceed to step 1465, at which the CPU 71sets a value of an imbalance occurrence flag XINB to (at) “1.” That is,the CPU 71 determines that an inter-cylinder air-fuel ratio imbalancestate has been occurring. Furthermore, the CPU 71 may turn on a warninglamp which is not shown. Note that the value of the imbalance occurrenceflag XINB is stored in the backup RAM 74. Next, the CPU 71 proceeds tostep 1495 to end the present routine tentatively.

In contrast, if the imbalance determination parameter X is equal to orsmaller than the imbalance determination threshold Xth when the CPU 71performs the process of step 1460, the CPU 71 makes a “No” determinationin step 1460 to proceed to step 1470, at which the CPU 71 sets the valueof the imbalance occurrence flag XINB to (at) “2.” That is, the CPU 71memorizes the “fact that it has been determined that the inter-cylinderair-fuel ratio imbalance state has not occurred according to the resultof the inter-cylinder air-fuel ratio imbalance determination.” Next, theCPU 71 proceeds to step 1495 to end the present routine tentatively.Note that step 1470 may be omitted.

Meanwhile, if the value of the parameter obtaining permission flagXkyoka is not “1” when the CPU 71 proceeds to step 1405, the CPU 71makes a “No” determination at step 1405 to proceed to step 1475.Subsequently, the CPU 71 sets (clears) the each of the values (e.g.,ΔAF, SAFD, SABF, Cn, etc.) to “0.” Next, the CPU 71 proceeds to step1480 to set the value of the sensor element temperature elevationrequest flag Xtupreq to (at) “0.” This decreases the heater duty Duty,so that the air-fuel ratio sensor element temperature is returned to theusual temperature (first temperature t1 serving as theparameter-non-obtaining-period-element-temperature). Thereafter, the CPU71 directly proceeds to step 1495 to end the present routinetentatively.

<Air-Fuel Ratio Sensor Heater Control>

Further, the CPU 71 executes an “air-fuel ratio sensor heater controlroutine” shown by a flowchart of FIG. 15 every time a predetermined timeelapses, in order to control the air-fuel ratio sensor elementtemperature.

Accordingly, when the predetermined timing comes, the CPU 71 startsprocessing from step 1500 in FIG. 15 to proceed to step 1510, at whichthe CPU 71 sets the target admittance Ytgt to (at) a usual value Ytujo.The target admittance Ytgt corresponds to a target value for theair-fuel ratio sensor element temperature. The usual value Ytujo is setto a value in such a manner that the air-fuel ratio sensor 67 becomesactivated, and the output value Vabyfs corresponds to a value whichcoincides with an air-fuel ratio of the exhaust gas when the air-fuelratio of the exhaust gas is stable. For example, the usual value Ytujois an admittance Y when the sensor element temperature is about 700° C.The air-fuel ratio sensor element temperature corresponding to the usualvalue Ytujo is “the usual temperature and the first temperature t1” asdescribed above.

Subsequently, the CPU 71 proceeds to step 1520 to determine whether ornot the sensor element temperature elevation request flag Xtupreq is“1.” When the sensor element temperature elevation request flag Xtupreqis “1”, the CPU 71 makes a “Yes” determination at step 1520 to proceedto step 1530, at which the CPU 71 sets the target admittance Ytgt to(at) a “value obtained by adding a predetermined positive value ΔY tothe usual value Ytujo.” That is, the CPU 71 makes the target admittanceYtgt larger than the usual value Ytujo. Thereafter, the CPU 71 proceedsto step 1540.

The “value obtained by adding the predetermined positive value ΔY to theusual value Ytujo” may also be referred to as an elevated value. Theelevated value is set to a value in such a manner that the air-fuelratio sensor 67 becomes activated, and the responsiveness of theair-fuel ratio sensor 67 is a “degree at which the output value Vabyfscan sufficiently follow the fluctuation of the air-fuel ratio sensor ofthe exhaust gas.” For example, the elevated value is an admittance Ywhen the sensor element temperature is about 850° C. The sensor elementtemperature corresponding to the elevated value is “the elevatedtemperature and the second temperature t2” as described above.

On the other hand, if the sensor element temperature elevation requestflag Xtupreq is not “1” (that is, it is “0”) when the CPU 71 executesthe process of step 1520, the CPU 71 makes a “No” determination at step1520 to directly proceed to step 1540.

The CPU 71, at step 1540, determines whether or not the actualadmittance Yact of (the solid electrolyte layer 671 of) the air-fuelratio sensor 67 is larger than a “value obtained by adding apredetermined positive value a to the target admittance Ytgt.”

When the condition in step 1540 is satisfied, the CPU 71 makes a “Yes”determination at step 1540 to proceed to step 1550, at which the CPU 71decreases the heater duty Duty by a predetermined amount AD.Subsequently, the CPU 71 proceeds to step 1560 to energize the heater678 based on the heater duty Duty. In this case, because the heater dutyis decreased, an amount of energy (current) supplied to the heater 678is decreased, so that the amount of heat generation by the heater 678 isdecreased. Consequently, the air-fuel ratio sensor element temperatureis decreased. Thereafter, the CPU 71 proceeds to step 1595 to end thepresent routine tentatively.

In contrast, if the actual admittance Yact is smaller than or equal tothe “value obtained by adding the predetermined positive value a to thetarget admittance Ytgt” when the CPU 71 executes the process of step1540, the CPU 71 makes a “No” determination at step 1540 to proceed tostep 1570. At step 1570, the CPU 71 determines whether or not the actualadmittance Yact is smaller than a “value obtained by subtracting thepredetermined positive value a from the target admittance Ytgt.”

When the condition in step 1570 is satisfied, the CPU 71 makes a “Yes”determination at step 1570 to proceed to step 1580, at which the CPU 71increases the heater duty Duty by the predetermined amount AD.Subsequently, the CPU 71 proceeds to step 1560 to energize the heater678 based on the heater duty Duty. In this case, because the heater dutyis increased, an amount of energy (current) supplied to the heater 678is increased, so that the amount of heat generation by the heater 678 isincreased. Consequently, the air-fuel ratio sensor element temperatureis elevated/increased/raised. Thereafter, the CPU 71 proceeds to step1595 to end the present routine tentatively.

In contrast, if the actual admittance Yact is larger than the “valueobtained by subtracting the predetermined positive value a from thetarget admittance Ytgt” when the CPU 71 executes the process of step1570, the CPU 71 makes a “No” determination at step 1570 to directlyproceed to step 1560. In this case, because the heater duty is notchanged, an amount of energy supplied to the heater 678 is not changed.Consequently, since the amount of heat generation by the heater 678 isnot changed, the air-fuel ratio sensor element temperature does notgreatly change. Thereafter, the CPU 71 proceeds to step 1595 to end thepresent routine tentatively.

In this manner, the actual admittance Yact is controlled within a ragein the vicinity of the target admittance Ytgt (the range between Ytgt-αand Ytgt+α) according to the heater control. In other words, theair-fuel ratio sensor element temperature is made coincide with a valuecorresponding to the target admittance Ytgt. Accordingly, the air-fuelratio sensor element temperature is maintained at a temperature in thevicinity of the usual temperature when the value of the sensor elementtemperature elevation request flag Xtupreq is “0”, and the air-fuelratio sensor element temperature is maintained at a temperature in thevicinity of the elevated temperature when the value of the sensorelement temperature elevation request flag Xtupreq is “1.”

As described above, the first determination apparatus is applied to themulti-cylinder internal combustion engine 10 having a plurality of thecylinders.

Further, the first determination apparatus comprises the air-fuel ratiosensor 67 including the sensor element section, a plurality of the fuelinjection valves 39, and heater control means for controlling the amountof heat generation of the heater 678 (the routine shown in FIG. 15).

Furthermore, the first determination apparatus comprises imbalancedetermining means which:

obtains, based on the output value Vabyfs of the air-fuel ratio sensor67, the imbalance determination parameter X which becomes larger as theair-fuel ratio variation/fluctuation of the “exhaust gas passing/flowingthrough the position at which the air-fuel ratio sensor 67 is disposed”becomes larger, in the period for/in which the predetermined parameterobtaining condition is being satisfied (parameter obtaining period inwhich the value of the parameter obtaining permission flag Xkyoka is“1”) (the “Yes” determination at step 1405 of FIG. 14, and steps fromstep 1420 to step 1455); determines that the inter-cylinder air-fuelratio imbalance state has occurred, when the obtained imbalancedetermination parameter X is larger than the predetermined imbalancedetermination threshold Xth (step 1460 and step 1465 of FIG. 14); anddetermines that the inter-cylinder air-fuel ratio imbalance state hasnot occurred, when the obtained imbalance determination parameter X issmaller than the imbalance determination threshold Xth (step 1460 andstep 1470, of FIG. 14).

Further, the imbalance determining means is configured so as to make theheater control means perform the “sensor element section temperatureelevation control to have/make the sensor element temperature for theparameter-obtaining-period be higher than the sensor element temperaturefor the period other than the parameter-obtaining-period (in which thesensor element section temperature is controlled to be the secondtemperature which is the elevated temperature) (the “Yes” determinationat step 1405 of FIG. 14, step 1410 of FIG. 14, the “Yes” determinationat step 1520 of FIG. 15, and step 1530 of FIG. 15).

Accordingly, the first determination apparatus can obtain the imbalancedetermination parameter X in the case where the responsiveness of theair-fuel ratio sensor 67 is good/superior. This allows the obtainedimbalance determination parameter X to become a value which accuratelyrepresents the inter-cylinder air-fuel ratio imbalance state(cylinder-by-cylinder air-fuel ratio difference). Consequently, thefirst determination apparatus can accurately perform the inter-cylinderair-fuel-ratio imbalance determination.

Further, the first determination apparatus maintains the air-fuel ratiosensor element temperature at the “temperature, which is equal to orhigher than the activation temperature, but which is relatively low (theusual temperature, the first temperature)” (the “No” determination atstep 1405 of FIG. 14, step 1480 of FIG. 14, and the “No” determinationat step 1520 of FIG. 15). Accordingly, it is possible to avoid that theair-fuel ratio sensor 67 deteriorates early, as compared to the case inwhich the air-fuel ratio sensor element temperature is always maintainedat the relatively high temperature (the elevated temperature, the secondtemperature).

Second Embodiment

Next, there will be described a determination apparatus according to asecond embodiment of the present invention (hereinafter simply referredto as the “second determination apparatus”).

The second determination apparatus firstly obtains the air-fuel ratiofluctuation indicating amount AFD as a tentative parameter X in a statein which the air-fuel ratio sensor element temperature is maintained atthe usual temperature (first temperature t1), compares the tentativeparameter X with a predetermined high-side threshold XHith, anddetermines that the inter-cylinder air-fuel ratio imbalance state hasbeen occurring when the tentative parameter X is larger than thehigh-side threshold XHith.

The high-side threshold XHith is set to (at) a relatively large valuewhich allows the apparatus to clearly determine that the “inter-cylinderair-fuel ratio imbalance state has been occurring” when the tentativeparameter X, which is obtained in the case in which the air-fuel ratiosensor element temperature is usual temperature, and thus, theresponsiveness of the air-fuel ratio sensor 67 is relatively low, islarger than the high-side threshold XHith.

On the other hand, when the tentative parameter X is smaller than thehigh-side threshold XHith, the second determination apparatus comparesthe tentative parameter X with a low-side threshold XLoth. The low-sidethreshold XLoth is smaller than the high-side threshold XHith by apredetermined amount. The low-side threshold XLoth is set to (at) arelatively small value which allows the apparatus to clearly determinethat the “inter-cylinder air-fuel ratio imbalance state has not beenoccurring” when the tentative parameter X is smaller than the low-sidethreshold XLoth. Further, the second determination apparatus determinesthat the “inter-cylinder air-fuel ratio imbalance state has not beenoccurring” when the tentative parameter X is smaller than the low-sidethreshold XLoth.

When the determination as to whether or not the inter-cylinder air-fuelratio imbalance state has been occurring is made using the tentativeparameter X as described above, the second determination apparatus doesnot perform the sensor element section temperature elevating control atleast until the current operation of the engine is stopped.

On the other hand, the second determination apparatus withholds (making)the determination as to whether or not the inter-cylinder air-fuel-ratioimbalance state has been occurring, when the tentative parameter X is“smaller than the high-side threshold XHith and larger than the low-sidethreshold XLoth”, and performs the sensor element section temperatureelevating control.

Thereafter, the second determination apparatus again obtains theair-fuel ratio fluctuation indicating amount AFD according to the methoddescribed above, in a state in which the air-fuel ratio sensor elementtemperature is elevated/increased to the elevated temperature (thesecond temperature t2). The obtained air-fuel ratio fluctuationindicating amount AFD is the imbalance determination parameter X, and isreferred to as a final parameter X, for convenience.

When the final parameter X is obtained, the second determinationapparatus compares the final parameter X with an imbalance determinationthreshold Xth (imbalance determination threshold Xth being equal to thehigh-side threshold XHith, in the second determination apparatus), anddetermines that the inter-cylinder air-fuel ratio imbalance state hasbeen occurring when the final parameter X is larger than the imbalancedetermination threshold Xth. In contrast, the second determinationapparatus determines that the inter-cylinder air-fuel ratio imbalancestate has not been occurring when the final parameter X is smaller thanthe imbalance determination threshold Xth. These are the principlesemployed by the second determination apparatus for the inter-cylinderair-fuel-ratio imbalance determination.

It should be noted that the imbalance determination threshold Xth may beset to (at) a value between the low-side threshold XLoth and thehigh-side threshold XHith. In other words, the high-side threshold XHithmay be equal to or larger than the imbalance determination thresholdXth, and the low-side threshold XLoth may be smaller than the imbalancedetermination threshold Xth.

(Actual Operation)

The CPU 71 of the second determination apparatus executes the routinesshown in FIGS. 12, 13, and 15, similarly to the first determinationapparatus. Further, the CPU 71 of the second determination apparatusexecutes routines shown by flowcharts of “FIGS. 16 and 18” every time apredetermined time (the sampling time ts) elapses. The routines shown inFIGS. 12, 13, and 15 have been already described. Accordingly, theroutines shown in FIGS. 16 and 18 will be described hereinafter. Itshould be noted that each step in FIGS. 16 and 18 at which the sameprocessing is performed as each step shown in FIG. 14 is given the samenumeral as one given to such step shown in FIG. 14.

It is assumed here that the parameter obtaining condition becomessatisfied in a state in which the imbalance determination has not beenmade yet since the current start of the engine 10, and the parameterobtaining permission flag Xkyoka is therefore set to (at) “1.” In thiscase, the CPU 71 makes a “Yes” determination at step 1405 shown in FIG.16 to determine whether or not a value of an imbalance determinationwithholding flag Xhoryu is “0.”

The value of the imbalance determination withholding flag Xhoryu is setto (at) “0” in the initial routine described above. Further, the valueof the imbalance determination withholding flag Xhoryu is set to (at)“1” after the imbalance determination is made based on the tentativeparameter X obtained while the air-fuel ratio sensor element temperatureis not elevated (i.e., while the air-fuel ratio sensor elementtemperature is maintained at the usual temperature) (and the value ofthe flag Xhoryu is set to (at) “1” when the imbalance determination iswithheld) (refer to step 1780 shown in FIG. 17 described later).

Accordingly, the value of the imbalance determination withholding flagXhoryu is “0.” This causes the CPU 71 to make a “Yes” determination atstep 1610, and to proceed to step 1620, at which the CPU 71 sets thevalue of the sensor element temperature elevation request flag Xtupreqto (at) “0.” As a result, the air-fuel ratio sensor element temperatureis maintained at the usual temperature (the air-fuel ratio sensorelement temperature when the actual admittance Yact is equal to theusual target admittance Ytgt=Ytujo”).

It should be noted that the value of the sensor element temperatureelevation request flag Xtupreq is set to (at) “0” in the initial routinedescribed above. Accordingly, the process of step 1620 at this stagedoes not change the value of the sensor element temperature elevationrequest flag Xtupreq substantially.

Thereafter, the CPU 71 obtains, as the “tentative parameter X”, theimbalance determination parameter X, by the processes of steps from step1420 to step 1455. That is, the air-fuel ratio fluctuation indicatingamount AFD is obtained in the case in which the air-fuel ratio sensorelement temperature is not elevated (the air-fuel ratio sensor elementtemperature is maintained at the usual temperature), and the air-fuelratio fluctuation indicating amount AFD is adopted as the imbalancedetermination parameter X (the tentative parameter X).

After the tentative parameter X is obtained at step 1455, the CPU 71proceeds to step 1640 to set a value of a parameter obtainmentcompletion flag Xobtain to (at) “1.” The value of the parameterobtainment completion flag Xobtain is also set to (at) “0” in theinitial routine described above. Thereafter, the CPU 71 proceeds to step1695 to end the present routine tentatively.

Meanwhile, the CPU 71 starts processing from step 1700 shown in FIG. 17,and proceeds to step 1710 to determine whether or not the present pointin time is immediately after the value of the parameter obtainmentcompletion flag Xobtain was changed from “0” to “1.” When thedetermining condition at step 1710 is not satisfied, the CPU 71 makes a“No” determination at step 1710 to directly proceed to step 1795 to endthe present routine tentatively.

Similarly, the CPU 71 starts processing from step 1800 shown in FIG. 18,and determines whether or not the present point in time is immediatelyafter the value of the parameter obtainment completion flag Xobtain waschanged from “0” to “1” at step 1810. When the determining condition atstep 1810 is not satisfied, the CPU 71 makes a “No” determination atstep 1810 to directly proceed to step 1895 to end the present routinetentatively.

Accordingly, when the tentative parameter X is obtained at step 1455 ofFIG. 16, and the value of the parameter obtainment completion flagXobtain is changed to “1” by the process of step 1640, the CPU 71 makesa “Yes” determination at step 1710 shown in FIG. 17 when the CPU 71proceeds to step 1710, and then proceeds to step 1720 to determinewhether or not the value of the imbalance determination withholding flagXhoryu (or the sensor element temperature elevation request flagXtupreq) is “0”

At the present time, the value of the imbalance determinationwithholding flag Xhoryu is “0”. Accordingly, the CPU 71 makes a “Yes”determination at step 1720 to proceed to step 1730, at which the CPU 71determines whether or not the value of the tentative parameter X islarger than a “predetermined high-side threshold XHith.”

When the value of the tentative parameter X is larger than the high-sidethreshold XHith, the CPU 71 makes a “Yes” determination at step 1730 toproceed to step 1740, at which the CPU 71 sets the value of theimbalance occurrence flag XINB to “1.” That is, the CPU 71 determinesthat the inter-cylinder air-fuel-ratio imbalance state has beenoccurring. At this time, the CPU 71 may turn on an unillustrated warninglamp. Thereafter, the CPU 71 proceeds to step 1795 to end the presentroutine tentatively.

In contrast, if the value of the tentative parameter X is smaller thanor equal to the high-side threshold XHith, the CPU 71 makes a “No”determination at step 1730 to proceed to step 1750, at which the CPU 71determines whether or not the value of the tentative parameter X issmaller than a “predetermined “low-side threshold XLoth.” The low-sidethreshold XLoth is smaller than the high-side threshold XHith.

When the tentative parameter X is smaller than the low-side thresholdXLoth, the CPU 71 makes a “Yes” determination at step 1750 to proceed tostep 1760, at which the CPU 71 sets the value of the value of theimbalance occurrence flag XINB to “2.” That is, the CPU 71 memorizes the“fact that it has been determined that the inter-cylinder air-fuel ratioimbalance state has not been occurring according to the result of theinter-cylinder air-fuel ratio imbalance determination.” Thereafter, theCPU 71 proceeds to step 1795 to end the present routine tentatively.

On the other hand, if the tentative parameter X is larger than or equalto the low-side threshold XLoth when the CPU 71 executes the process ofstep 1750, the CPU 71 withholds the imbalance determination. That is,the CPU 71 withholds making a conclusion as to whether or not theinter-cylinder air-fuel-ratio imbalance state has occurred. Thereafter,the CPU 71 elevates the air-fuel ratio sensor element temperature toagain perform the obtainment of the imbalance parameter X (air-fuelratio fluctuation indicating amount AFD) and the imbalancedetermination.

More specifically, when the tentative parameter X is larger than orequal to the low-side threshold XLoth, the CPU 71 makes a “No”determination at step 1750 to proceed to step 1770, at which the CPU 71sets the value of the parameter obtainment completion flag Xobtain to(at) “0.” Subsequently, the CPU 71 proceeds to step 1780 to set thevalue of the imbalance determination withholding flag Xhoryu to (at)“1.” Thereafter, the CPU 71 proceeds to step 1790 to set (or clear) eachof the values used for obtaining the imbalance determination parameter X(e.g., ΔAF, SAFD, Cn, AveΔAF, Save, Cs, and so on) to (at) “0”.Subsequently, the CPU 71 proceeds to step 1795 to end the presentroutine tentatively.

After that, when the CPU 71 starts processing the routine shown in FIG.16 to proceed to step 1610, the CPU 71 makes a “No” determination atstep 1610 since the value of the imbalance determination withholdingflag Xhoryu is set to (at) “1”, and proceeds to step 1630, at which theCPU 71 sets the value of the sensor element temperature elevationrequest flag Xtupreq to (at) “1.”

When the value of the sensor element temperature elevation request flagXtupreq is set to (at) “1”, the target admittance Ytgt is set to theelevated value (the value obtained by adding the predetermined positivevalue ΔY to the usual value Ytujo) at step 1530 shown in FIG. 15. Thisimproves/increases the responsiveness of the air-fuel ratio sensorsufficiently, and thus, the accurate imbalance determination parameter Xcan be obtained.

Further, the CPU 71 executes the processes of steps from step 1415 tostep 1445, shown in FIG. 16. Accordingly, when the counter Cs becomesequal to or larger than the threshold value Csth, the CPU 71 proceedsfrom step 1445 to step 1455 to again obtain the imbalance determinationparameter X.

The imbalance determination parameter X is a parameter obtained whilethe air-fuel ratio sensor element temperature is elevated, and is alsoreferred to as the “final parameter” for convenience.

Subsequently, the CPU 71 sets the value of the parameter obtainmentcompletion flag Xobtain to (at) “1” at step 1640, and proceeds to step1695 to end the present routine tentatively.

Consequently, the value of the parameter obtainment completion flagXobtain is changed from “0” to “1.” Accordingly, the CPU 71 makes a“Yes” determination at step 1710 shown in FIG. 17 when the CPU 71proceeds to step 1710, and proceeds to step 1720. At this moment, thevalue of the imbalance determination withholding flag Xhoryu is “1.” TheCPU 71 therefore makes a “No” determination at step 1720 to directlyproceed to step 1795 to end the present routine tentatively.

Meanwhile, when the CPU 71 proceeds to step 1810 shown in FIG. 18 atthis stage, the CPU 71 makes a “Yes” determination at step 1810 toproceed to step 1820. The CPU 71 determines whether or not the value ofthe imbalance determination withholding flag Xhoryu is “1” at step 1820.Here, the value of the imbalance determination withholding flag Xhoryuis “1.” Accordingly, the CPU 71 makes a “Yes” determination at step 1820to proceed to step 1830, at which the CPU 71 determines whether or notthe final parameter X is larger than the imbalance determinationthreshold Xth (which is equal to the high-side threshold XHith, in thepresent example).

When the final parameter X is larger than the imbalance determinationthreshold Xth, the CPU 71 makes a “Yes” determination at step 1830 toproceed to step 1840, at which the CPU 71 b sets the value of theimbalance occurrence flag XINB to “1.” That is, the CPU 71 determinesthat the inter-cylinder air-fuel ratio imbalance state has beenoccurring. Thereafter, the CPU 71 proceeds to step 1860.

To the contrary, if the final parameter X is smaller than or equal tothe imbalance determination threshold Xth when the CPU 71 executes theprocess of step 1830, the CPU 71 makes a “No” determination at step 1830to proceed to step 1850, at which the CPU 71 b sets the value of theimbalance occurrence flag XINB to “2.” That is, the CPU 71 memorizes the“fact that it has been determined that the inter-cylinder air-fuel ratioimbalance state has not been occurring according to the result of theinter-cylinder air-fuel ratio imbalance determination.” Subsequently,the CPU 71 proceeds to step 1860.

The CPU 71 sets the value of the sensor element temperature elevationrequest flag Xtupreq to (at) “0” at step 1860, and proceeds to step 1895to end the present routine tentatively. As a result, the air-fuel ratiosensor element temperature is returned to the usual temperature.

It should be noted that, if the value of the imbalance determinationwithholding flag Xhoryu is “0” when the CPU 71 proceeds to step 1820shown in FIG. 18, the CPU 71 makes a “No” determination at step 1820 todirectly proceed to step 1895 to end the present routine tentatively.

As described above, the imbalance determining means of the seconddetermination apparatus:

obtains, based on the output value Vabyfs of the air-fuel ratio sensor67, the imbalance determination parameter X as the tentative parameter Xbefore having the heater control means perform the “sensor elementsection temperature elevating control” in/during theparameter-obtaining-period (the parameter obtaining permission flagXkyoka=1) (step 1610, step 1620, and steps from step 1420 to step 1455,in FIG. 16),

determines that the inter-cylinder air-fuel ratio imbalance state hasbeen occurring, when the obtained tentative parameter X is larger thanthe “high-side threshold Hith” (step 1730 and step 1740, in FIG. 17),and

determines that the inter-cylinder air-fuel ratio imbalance state hasnot been occurring, when the obtained tentative parameter X is smallerthan the “low-side threshold XLoth which is smaller by the predeterminedvalue than the high-side threshold XHith” (step 1750 and step 1760, inFIG. 17).

Further, the imbalance determining means:

withholds (making) the determination as to whether or not theinter-cylinder air-fuel-ratio imbalance state has occurred, when theobtained tentative parameter X is smaller than the high-side thresholdXHith and is larger than the low-side threshold XLoth (refer to the “No”determinations in both step 1730 and step 1750, in FIG. 17),

has the heater control means perform the sensor element sectiontemperature elevating control in/during the parameter-obtaining-period(the parameter obtaining permission flag Xkyoka=1) in the case in whichthe determination as to whether or not the inter-cylinder air-fuel-ratioimbalance state has occurred is being withheld (the imbalancedetermination withholding flag Xhoryu=1) (step 1780 in FIG. 17, step1610 and step 1630 in FIG. 16, step 1520 and step 1530, in FIG. 15), andobtain, based on the output value Vabyfs of the air-fuel ratio sensor67, the imbalance determination parameter X as the final parameter X(steps from step 1420 to step 1455, in FIG. 16); and

determines that the inter-cylinder air-fuel-ratio imbalance state hasoccurred when the obtained final parameter X is larger than theimbalance determination threshold Xth (step 1830 and step 1840, in FIG.18), and determines that the inter-cylinder air-fuel-ratio imbalancestate has not occurred when the obtained final parameter X is smallerthan the imbalance determination threshold Xth (step 1830 and step 1850,in FIG. 18).

According to the second determination apparatus, the sensor elementsection temperature elevating control is not performed, when it ispossible to make a clear determination as to “whether or not theinter-cylinder air-fuel-ratio imbalance state has occurred” based on theimbalance determination parameter (the tentative parameter) obtainedwhile the responsiveness of the air-fuel ratio sensor is relatively low.Consequently, chances/frequency of elevating/raising the air-fuel ratiosensor element temperature to the relatively high temperature (theelevated temperature) for the imbalance determination is decreased, andthus, it can be avoided that the deterioration of the air-fuel ratiosensor 67 is accelerated.

Further, according to the second determination apparatus, in the case inwhich the determination as to whether or not the inter-cylinderair-fuel-ratio imbalance state has occurred is withheld, the air-fuelratio sensor element temperature is elevated (raised) to the elevatedtemperature, and thus, the imbalance determination parameter (the finalparameter) can be obtained while the responsiveness of the air-fuelratio sensor 67 is high. Accordingly, even in the case in which it isnot possible to clearly determine whether or not the inter-cylinderair-fuel-ratio imbalance state has occurred using the tentativeparameter, the imbalance determination can be performed accurately usingthe final parameter.

Third Embodiment

Next, there will be described a determination apparatus according to athird embodiment of the present invention (hereinafter simply referredto as the “third determination apparatus”).

The third determination apparatus is different from the firstdetermination apparatus only in that the third determination apparatusshortens a delay time Tdelayth as the intake air flow rate Ga becomeslarger, the delay time Tdelayth being a time (period) from a point intime the amount of energy supplied to the heater 678 is increased inorder to elevate the air-fuel ratio sensor element temperature (i.e., apoint in time at which the apparatus starts having the heater controlmeans perform the sensor element section temperature elevating control)to a point in time the base indicating amount (detected air-fuel ratiochange rate ΔAF) which is the base data for the air-fuel ratiofluctuation indicating amount AFD (imbalance determination parameter)starts to be obtained.

(Actual Operation)

The CPU 71 of the third determination apparatus determines the delaytime Tdelayth based on the intake air flow rate Ga when the CPU 71proceeds to step 1415 shown in FIG. 14. More specifically, at step 1415,the CPU 71 determines the delay time Tdelayth by applying the intake airflow rate Ga at that point in time to a delay time table MapTdelayth(Ga)shown in FIG. 19.

According to the delay time table MapTdelayth(Ga), the delay timeTdelayth is determined in such a manner that the delay time Tdelaythbecomes shorter as the intake air flow rate Ga becomes larger. This isbecause the air-fuel ratio sensor element temperature more rapidlybecomes higher when the intake air flow rate Ga becomes larger, sincethe exhaust gas temperature is higher as the intake air flow rate Ga islarger.

In this manner, the third determination apparatus changes the delay timeTdelayth based on the intake air flow rate Ga, and thus, the delay timeTdelayth can be set to be as short as possible. As a result, chances toobtain the air-fuel ratio fluctuation indicating amount AFD (theimbalance determination parameter) can be increased.

It should be noted that, similarly to the third determination apparatus,“changing the delay time Tdelayth based on the intake air flow rate Ga”can be applied not only to the first embodiment but also to “the secondembodiment and another embodiments described later.” Further, the delaytime Tdelayth may be determined based on “the engine load KL, theexhaust gas temperature (estimated or actually measured temperature),and the like” in place of the intake air flow rate Ga. That is, thedelay time Tdelayth may be determined based on an operating parameterrelating to (associated with) the exhaust gas temperature. For example,in a determination apparatus equipped with an exhaust gas temperaturesensor, the delay time Tdelayth may be set so as to be shorter as theexhaust gas temperature measured by the exhaust gas temperature sensoris higher. Alternatively, the delay time Tdelayth may be set so as to beshorter as the load (KL) of the engine 10 is higher.

Fourth Embodiment

Next, there will be described a determination apparatus according to afourth embodiment of the present invention (hereinafter simply referredto as the “fourth determination apparatus”).

The fourth determination apparatus is different from the firstdetermination apparatus only in that the fourth determination apparatusstarts the sensor element section temperature elevating controlimmediately after the warming up of the engine 10 has completed afterthe start of the engine (i.e., at the completion of the warming-up),even when the parameter obtaining condition is not satisfied.

(Actual Operation)

The CPU 71 of the fourth determination apparatus executes the routinesshown in FIGS. 12 and 13, similarly to the CPU 71 of the firstdetermination apparatus. Further, the CPU 71 of the fourth determinationapparatus executes routines shown by flowcharts of FIGS. 20 and 22 everytime a predetermined time (the sampling time ts) elapses. The routinesshown in FIGS. 12 and 13 have been already described. Accordingly, theroutines shown in FIGS. 20 and 22 will be described hereinafter. Itshould be noted that each step in FIGS. 20 and 22 at which the sameprocessing is performed as each step which has been already described isgiven the same numeral as one given to such step.

It is assumed that the present time is immediately after the engine 10was started. Usually, warming up of the engine 10 has not completed atthe point in time immediately after the engine 10 was started (that is,the state is not the completion of the warming-up).

When the predetermined timing comes, the CPU 71 starts processing fromstep 2000 shown in FIG. 20 to proceed to step 2010, at which the CPU 71determines whether or not the state of the engine 10 reaches thewarming-up completion state after the current start of the engine. Forexample, the CPU 71 determines whether or not the state of the engine 10reaches the warming-up completion state by determining whether or notthe cooling water temperature THW is equal to or higher than a“threshold cooling water temperature THWth which is a cooling watertemperature at the warming-up completion state.” Further, the CPU 71 maydetermine whether or not the state of the engine 10 reaches thewarming-up completion state by obtaining threshold air flow rateintegrated value SGath which becomes smaller as the cooling watertemperature THW at the start of the engine 10 becomes higher, obtainingintegrated value SGa of the intake air flow rate Ga after the start ofthe engine 10, and determining whether or not the integrated value SGabecomes higher than the threshold air flow rate integrated value SGath,for instance.

According to the assumption described above, since the present point intime is immediately after the start of the engine, the state of theengine 10 has not reached the warming-up completion state. The CPU 71therefore makes a “No” determination at step 2010 to proceed to step2020, at which the CPU 71 sets the value of the sensor elementtemperature elevation request flag Xtupreq to (at) “0.” Thereafter, theCPU 71 proceeds to step 2095 to end the present routine tentatively.

Further, the CPU 71 starts processing from step 2100 shown in FIG. 21 ata predetermined timing. The “air-fuel ratio sensor heater controlroutine” shown in FIG. 21 is the same as the “air-fuel ratio sensorheater control routine” shown in FIG. 15 executed by the CPU 71 of thefirst determination apparatus.

In addition, the value of the sensor element temperature elevationrequest flag Xtupreq is set to (at) “0” at the present point in time.Accordingly, the CPU 71 executes processes of step 1510 and step 1520,and thereafter proceeds to steps following step 1540 without executingthe process of step 1530. Consequently, the heater 678 is energized insuch a manner that the air-fuel ratio sensor element temperaturecoincides with the usual temperature (i.e., the actual admittance Yactcoincides with the usual value Ytujo).

Further, the CPU 71 starts processing from step 2200 shown in FIG. 22 ata predetermined timing. The “inter-cylinder air-fuel-ratio imbalancedetermination routine” shown in FIG. 22 is the same as the“inter-cylinder air-fuel-ratio imbalance determination routine” shown inFIG. 14 executed by the CPU 71 of the first determination apparatus,except that “step 1410 and step 1480” are omitted/deleted from theroutine shown in FIG. 14.

Accordingly, if the value of the parameter obtaining permission flagXkyoka is not “1” (i.e., the parameter obtaining condition is notsatisfied) when the CPU 71 executes the process of step 1405 shown inFIG. 22, the CPU 71 makes a “No” determination at step 1405 to proceedto step 1475, at which the CPU 71 clears each of the values. Thereafter,the CPU 71 proceeds to step 2295 to end the present routine tentatively.

In contrast, if the value of the parameter obtaining permission flagXkyoka is “1” (i.e., the parameter obtaining condition is satisfied)when the CPU 71 executes the process of step 1405, the CPU 71 makes a“Yes” determination at step 1405 to proceed to step 1415. At step 1415,the CPU 71 determines whether or not a delay time Tdelayth has elapsedsince a point in time at which the value of the sensor elementtemperature elevation request flag Xtupreq was changed from “0” to “1.”

At the present point in time, the value of the sensor elementtemperature elevation request flag Xtupreq is set to (at) “0” (refer tostep 2020 shown in FIG. 20 described above). The CPU 71 therefore makesa “No” determination at step 1415 shown in FIG. 22 to directly proceedto step 2295 so as to end the present routine tentatively.

Thereafter, the state of the engine 10 reaches the warming-up completionstate when a predetermined time elapses. At this moment, when the CPU 71executes the process of step 2020 shown in FIG. 20, the CPU 71 makes a“Yes” determination at step 2010 to proceed to step 2030, at which theCPU 71 determines whether or not “obtainment of the imbalancedetermination parameter X has not been completed (the imbalancedetermination parameter has not been obtained) since the current startof the engine 10”.

The present point in time is immediately after a point in time at whichthe engine 10 reached the warming-up completion state after the start ofthe engine 10. Accordingly, the imbalance determination parameter X hasnot been obtained yet, and the CPU 71 therefore makes a “Yes”determination at step 2030 to proceed to step 2040, at which the CPU 71sets the value of the sensor element temperature elevation request flagXtupreq to “1.” Thereafter, the CPU 71 proceeds to step 2095 to end thepresent routine tentatively.

In this state, since the value of the sensor element temperatureelevation request flag Xtupreq is set to (at) “1”, when the CPU 71starts processing the routine shown in FIG. 21 from step 2100, the CPU71 proceeds to step 2100, step 1510, step 1520, and then, step 1530, atwhich the CPU 71 sets the target admittance Ytgt to (at) the “value(elevated value) obtained by adding the predetermined positive value ΔYto the usual value Ytujo.” Subsequently, the CPU 71 proceeds to stepsfollowing step 1540. Consequently, the heater 678 is energized in such amanner that the air-fuel ratio sensor element temperature coincides withthe elevated temperature (the actual admittance Yact coincides with thevalue obtained by adding the predetermined positive value ΔY to theusual value Ytujo).

Under this state, if the value of the parameter obtaining permissionflag Xkyoka is set to (at) “1” owing to the satisfaction of theparameter obtaining condition, the CPU 71 makes a “Yes” determination atstep 1405 shown in FIG. 22 when the CPU 71 proceeds to step 1405, andthen proceeds to step 1415.

At this moment, if the delay time Tdelayth has not elapsed since thepoint in time at which the value of the sensor element temperatureelevation request flag Xtupreq was changed from “0” to “1”, the CPU 71makes a “No” determination at step 1415 to directly proceed to step 2295to end the present routine tentatively.

In contrast, at a point in time at which the CPU 71 executes the processof step 1415, if the delay time Tdelayth has elapsed since the point intime at which the value of the sensor element temperature elevationrequest flag Xtupreq was changed from “0” to “1”, the CPU 71 proceedsfrom step 1415 to step 1420.

As a result, the air-fuel ratio fluctuation indicating amount AFD andthe imbalance determination parameter X are obtained while the air-fuelratio sensor element temperature is at the elevated temperature.Further, the processes following step 1460 shown in FIG. 22, theimbalance determination is made based on the comparison result betweenthe imbalance determination parameter X and the imbalance determinationthreshold Xth.

Further, when the CPU 71 executes the process of step 2030 shown in FIG.20 after the completion of the obtainment of the imbalance determinationparameter X owing to the processes of step 1450 and step 1455 shown inFIG. 22, the CPU 71 makes a “No” determination at step 2030 so as toproceed to step 2020. That is, the sensor element temperature elevationrequest flag Xtupreq is set/returned to “0” immediately after theimbalance determination parameter X is obtained and the imbalancedetermination is completed. As a result, the air-fuel ratio sensorelement temperature is decreased to the usual temperature immediatelyafter the completion of the obtainment of the imbalance determinationparameter X.

As described above, the fourth determination apparatus comprisesimbalance determining means which is configured so as to have/make theheater control means start to perform the sensor element sectiontemperature elevating control at the point in time at which thewarming-up of the engine 10 is completed after the start of the engine10 (step 2010, step 2040, and step 2040, shown in FIG. 20), and so as tohave/make the heater control means finish/end the sensor element sectiontemperature elevating control at the point in time at which obtainingthe imbalance determination parameter X is completed (step 2030 and step2020, shown in FIG. 20).

It requires some time for the air-fuel ratio sensor element temperatureto actually increase/becomes higher after the start of the execution ofthe sensor element section temperature elevating control. Accordingly,if the sensor element section temperature elevating control is startedafter the parameter obtaining condition becomes satisfied, obtaining thebase indicating amount (detected air-fuel ratio change rate ΔAF) whichis the base data for the imbalance determination parameter X can not bestarted until the air-fuel ratio sensor element temperature becomessufficiently high. Alternatively, if the base indicating amount(detected air-fuel ratio change rate ΔAF) is started to be obtained atthe same time of the start of performing the sensor element sectiontemperature elevating control after the satisfaction of the parameterobtaining condition, the base indicating amount (and accordingly, theair-fuel ratio fluctuation indicating amount AFD and the imbalancedetermination parameter X) can not become a value which sufficientlyaccurately represents the cylinder-by-cylinder air-fuel ratiodifference, because the responsiveness of the air-fuel ratio sensor 67is not sufficiently high.

Moreover, for example, according to the first determination apparatus,if the parameter obtaining condition becomes unsatisfied in a periodfrom the start of the execution of the sensor element sectiontemperature elevating control to a point in time at which the air-fuelratio sensor element temperature becomes sufficiently high, the sensorelement section temperature elevating control is stopped. Consequently,chances/frequency to obtain the imbalance determination parameter maydecrease.

On the other hand, in a case in which the engine 10 has not been warmedup yet after the start of the engine 10, moisture in the exhaust gas iseasily cooled down by members constituting the engine 10, the outerprotective cover 67 b, or the like, to thereby be likely to form waterdroplets. In a case in which the water droplets adhere to the air-fuelratio sensor 67 (hereinafter, this is expressed as “the air-fuel ratiosensor gets wet with water”), if the temperature of the “air-fuel ratiodetecting section including the sensor element section” is elevated bythe sensor element section temperature elevating control, a greattemperature unevenness in the air-fuel ratio detecting section of theair-fuel ratio sensor 67 occurs, and thus, the air-fuel ratio detectingsection may crack/dunt (be broken). Accordingly, it is not preferable toperform the sensor element section temperature elevating controlimmediately after the start of the engine.

In view of the above, the imbalance determining means of the fourthdetermination apparatus starts the sensor element section temperatureelevating control at the point in time at which the warming up of theengine 10 has been completed. Accordingly, the air-fuel ratio sensorelement temperature is elevated in a state in which it is unlikely thatthe air-fuel ratio sensor gets wet with water. Therefore, the fourthdetermination apparatus can increase chances in which the air-fuel ratiosensor element temperature is sufficiently high when the parameterobtaining condition becomes satisfied while avoiding the state in whichthe air-fuel ratio sensor 67 is broken due to getting wet with water.Consequently, the fourth determination apparatus can increase chances toobtain the imbalance determination parameter X which has a high accuracyand increase chances to perform the imbalance determination using suchan imbalance determination parameter.

Fifth Embodiment

Next, there will be described a determination apparatus according to afifth embodiment of the present invention (hereinafter simply referredto as the “fifth determination apparatus”).

FIG. 23 is a graph showing a relation between the air-fuel ratio sensorelement temperature and the admittance Y of the solid electrolyte layer671. In FIG. 23, a solid line Y1 indicates the admittance Y when theair-fuel ratio sensor 67 has not deteriorated with age (for example,when the air-fuel ratio sensor 67 is brand new), and a broken line Y2indicates the admittance Y when the air-fuel ratio sensor 67 hasdeteriorated with age (for example, when the air-fuel ratio sensor 67has been used for a relatively long time).

As understood from FIG. 23, when the admittance Y is a “certain specificvalue”, the element temperature of the air-fuel ratio sensor 67 whichhas deteriorated with age is higher than the element temperature of theair-fuel ratio sensor 67 which has not deteriorated with age. Meanwhile,the electric controller 70 control the amount of energy supplied to theheater 678 in such a manner that the actual admittance Yact of theair-fuel ratio sensor 67 coincides with the target admittance Ytgt.

From the above fact, it is understood that the element temperature ofthe air-fuel ratio sensor 67 which has deteriorated with age issufficiently high even when the target admittance Ytgt is maintained atthe usual value Ytujo. That is, in the example shown in FIG. 23, theair-fuel ratio sensor element temperature is about 800° C. when theactual admittance Yact of the air-fuel ratio sensor 67 which has notdeteriorated with age is made equal to the usual value Ytujo, and theair-fuel ratio sensor element temperature is about 870° C. when theactual admittance Yact of the air-fuel ratio sensor 67 which has notdeteriorated with age is made equal to the elevated value (Ytujo+ΔY). Incontrast, the air-fuel ratio sensor element temperature is about 870° C.when the actual admittance Yact of the air-fuel ratio sensor 67 whichhas deteriorated with age is made equal to the usual value Ytujo.

In other words, the element temperature of the air-fuel ratio sensor 67which has deteriorated with age while the target admittance Ytgt is setto (at) the usual value Ytujo is roughly equal to the elementtemperature of the air-fuel ratio sensor 67 which has not deterioratedwith age while the target admittance Ytgt is set to (at) the elevatedvalue (Ytujo+ΔY). Accordingly, it can be said that the responsiveness ofthe air-fuel ratio sensor 67 which has deteriorated with age issufficiently high even if the target admittance Ytgt is set to (at) theusual value Ytujo.

In view of the above, if the air-fuel ratio sensor 67 has notdeteriorated with age, the fifth determination apparatus performs thesensor element section temperature elevating control when obtaining theair-fuel ratio fluctuation indicating amount AFD and the imbalancedetermination parameter X, similarly to the first determinationapparatus. On the other hand, if the air-fuel ratio sensor 67 hasdeteriorated with age, the fifth determination apparatus does notperform the sensor element section temperature elevating control whenobtaining the air-fuel ratio fluctuation indicating amount AFD and theimbalance determination parameter X.

(Actual Operation)

The CPU 71 of the fifth determination apparatus executes the routinesshown in FIGS. 12, 13, and 15, similarly to the CPU 71 of the firstdetermination apparatus. Further, the CPU 71 of the fifth determinationapparatus executes routines shown by flowcharts of FIGS. 24 and 25 everytime a predetermined time (the sampling time ts) elapses. The routinesshown in FIGS. 12, 13, and 15 have been already described. Accordingly,the operation of the CPU 71 will be described hereinafter with referenceto the routines shown in FIGS. 24 and 25. It should be noted that eachstep in FIGS. 24 and 25 at which the same processing is performed aseach step which has been already described is given the same numeral asone given to such step.

When the CPU 71 starts processing from step 2400 shown in FIG. 24 toproceed to step 1405, the CPU 71 makes a “No” determination at step 1405if the value of the parameter obtaining permission flag Xkyoka is “0”,so that the CPU 71 executes the processes of step 1475 and step 1480,and directly proceeds to step 2495 to end the present routinetentatively.

In contrast, if the value of the parameter obtaining permission flagXkyoka is “1” when the CPU 71 executes the process of step 1405, the CPU71 makes a “Yes” determination at step 1405.

Thereafter, at step 2410, the CPU 71 determines whether or not theair-fuel ratio sensor 67 has deteriorated with age (i.e., it hasdeteriorated as compared to a brand new sensor) using any one of thefollowing ways. That is, it is determined whether or not the air-fuelratio sensor 67 is an aged sensor.

(Method 1 for Determination of Deterioration with Age)

The CPU 71 obtains a “duty integrated value SD” which is a valueobtained by integrating/accumulating a value of the heater duty Dutywhich is the instruction signal supplied to the heater 678.” Theintegrated value SD is stored in the backup RAM 74. That is, theintegrated value SD is an integrated value of the heater duty Duty for aperiod from a point in time when the air-fuel ratio sensor 67 was abrand new one to a present point in time. Thereafter, the CPU 71determines that the air-fuel ratio sensor has deteriorated with age,when the integrated value SD becomes equal to or larger than apredetermined deterioration determination threshold SDth.

(Method 2 for Determination of Deterioration with Age)

The CPU 71 obtains a time integration value SIh of an actual currentvalue (heater current) Ih flowing through the heater 678. The timeintegration value SIh is stored in the backup RAM 74. That is, the timeintegration value SIh is an integrated/accumulated value of the heatercurrent Ih for a period from a point in time when the air-fuel ratiosensor was brand new and the present point in time. Thereafter, the CPU71 determines that the air-fuel ratio sensor 67 has deteriorated withage when the time integration value SIh is equal to or larger than apredetermined deterioration determination threshold SIhth.

(Method 3 for Determination of Deterioration with Age)

The CPU 71 obtains a time integration value SGa of the intake air flowrate Ga. The time integration value SGa is stored in the backup RAM 74.That is, the time integration value SGa is an integrated/accumulatedvalue of the intake air flow rate Ga for a period from a point in timewhen the air-fuel ratio sensor was brand new and the present point intime. Thereafter, the CPU 71 determines that the air-fuel ratio sensor67 has deteriorated with age when the time integration value SGa isequal to or larger than a predetermined deterioration determinationthreshold SrGath.

(Method 4 for Determination of Deterioration with Age)

The CPU 71 obtains an integrated/accumulated running distance SS of thevehicle on which the engine 10 is mounted. The integrated runningdistance SS is stored in the backup RAM 74. That is, the integratedrunning distance SS is a “total running distance of the vehicle” for aperiod from a point in time when the air-fuel ratio sensor was brand newand the present point in time. Thereafter, the CPU 71 determines thatthe air-fuel ratio sensor 67 has deteriorated with age when theintegrated running distance SS is equal to or larger than apredetermined deterioration determination threshold SSth.

It is assumed here that the air-fuel ratio sensor 67 is substantiallynew, and therefore, has not deteriorated. In this case, the CPU 71 makesa “No” determination at step 2410 to proceed to step 2420, at which theCPU 71 sets the value of the sensor element temperature elevationrequest flag Xtupreq to (at) “1.” Consequently, by the execution of theroutine shown in FIG. 15, the sensor element section temperatureelevating control is performed.

Subsequently, the CPU 71 proceeds to step 1415 to determine whether ornot the delay time Tdelay has elapsed since the value of the sensorelement temperature elevation request flag Xtupreq was changed from “0”to “1.” When the delay time Tdelay has not elapsed since the value ofthe sensor element temperature elevation request flag Xtupreq waschanged from “0” to “1”, the CPU 71 makes a “No” determination at step1415 to directly proceed to step 2495, at which the CPU 71 ends thepresent routine tentatively.

On the other hand, if the delay time Tdelay has elapsed since the valueof the sensor element temperature elevation request flag Xtupreq waschanged from “0” to “1” when the CPU 71 executes the process of step1415 shown in FIG. 24, the CPU 71 proceeds from step 1415 to stepsfollowing step 1420. Consequently, the air-fuel ratio fluctuationindicating amount AFD is obtained at step 1450, and the imbalancedetermination parameter X is obtained at step 1455. Further, the valueof the parameter obtainment completion flag Xobtain is set to (at) “1”at step 1640.

Meanwhile, the CPU 71 starts processing the routine from step 2500 shownin FIG. 25 every time a predetermined time elapses, and alwaysdetermines whether or not the value of the parameter obtainmentcompletion flag Xobtain is changed from “0” to “1.”

Accordingly, when the value of the parameter obtainment completion flagXobtain is changed from “0” to “1” at step 1640 shown in FIG. 24, theCPU 71 makes a “Yes” determination at step 1810 shown in FIG. 25 toproceed to steps following step 1830, so that the CPU 71 determineswhether or not the inter-cylinder air-fuel-ratio imbalance state hasoccurred based on the comparison result between the imbalancedetermination parameter X and the imbalance determination threshold Xth.That is, the CPU 71 determines that the inter-cylinder air-fuel-ratioimbalance state has been occurring, when the imbalance determinationparameter X is larger than the imbalance determination threshold Xth(step 1830 and step 1840). Further, the CPU 71 determines that theinter-cylinder air-fuel-ratio imbalance state has not occurred, when theimbalance determination parameter X is smaller than or equal to theimbalance determination threshold Xth (step 1830 and step 1850).

Thereafter, the CPU 71 sets the value of the sensor element temperatureelevation request flag Xtupreq to (at) “0” at step 1860, and proceeds tostep 2995 to end the present routine tentatively. This stops the sensorelement section temperature elevating control.

As described above, when the air-fuel ratio sensor 67 has notdeteriorated with age, the imbalance determination parameter X isobtained under the state in which the sensor element section temperatureelevating control is being performed, and the inter-cylinderair-fuel-ratio imbalance determination is carried out using theimbalance determination parameter X.

Next, there will be described the case in which the air-fuel ratiosensor 67 has deteriorated with age. In this case, the CPU 71 makes a“Yes” determination at step 2410 shown in FIG. 24 when the CPU 71proceeds to step 2410. Thereafter, the CPU 71 proceeds to step 2430 toset the value of the sensor element temperature elevation request flagXtupreq to (at) “0.” It should be noted that, in actuality, since thevalue of the sensor element temperature elevation request flag Xtupreqis set to (at) “0” in the initial routine described above, the CPU 71does not change the value of the sensor element temperature elevationrequest flag Xtupreq at step 2430. Consequently, the sensor elementsection temperature elevating control is not carried out.

Thereafter, the CPU 71 proceeds to steps following step 1420.Consequently, the air-fuel ratio fluctuation indicating amount AFD isobtained at step 1450, and the imbalance determination parameter X isobtained at step 1455. Further, the value of the parameter obtainmentcompletion flag Xobtain is set to (at) “1” at step 1640.

When the value of the parameter obtainment completion flag Xobtain isstep to (at) “1” at step 1640 shown in FIG. 24, the CPU 71 makes a “Yes”determination at step 1810 shown in FIG. 25 to proceed to stepsfollowing step 1830, so that the CPU 71 performs the above describedimbalance determination based on the comparison result between theimbalance determination parameter X and the imbalance determinationthreshold Xth. Thereafter, the CPU 71 proceeds to step 2595 via step1860 to end the present routine tentatively.

As described above, according to the fifth determination apparatus, whenthe air-fuel ratio sensor 67 has deteriorated with age, the imbalancedetermination parameter X is obtained without performing the sensorelement section temperature elevating control, and the inter-cylinderair-fuel-ratio imbalance determination is carried out using theimbalance determination parameter X.

That is, similarly to the heater control means of the first to fourthdetermination apparatus, the heater control means of the fifthdetermination apparatus controls amount of heat generation of the heaterin such a manner that the difference between the value corresponding tothe actual admittance Yact (e.g. the actual admittance) of the solidelectrolyte layer 671 and the target value (the target admittance Ytgt)becomes smaller (refer to the routine shown in FIG. 15). Further, theheater control means is configured so as to realize the sensor elementsection temperature elevating control by making the target value (thetarget admittance Ytgt) during the sensor element section temperatureelevating control is being performed different from (larger than) thetarget value during the sensor element section temperature elevatingcontrol is not being performed (steps from step 1510 to step 1530, shownin FIG. 15).

Further, the imbalance determining means of the fifth determinationapparatus is configured so as to:

include deterioration-with-age-occurrence determining means fordetermining whether or not the air-fuel ratio has deteriorated with age(step 2410 shown in FIG. 24); and

obtain, when it is determined that the air-fuel ratio has deterioratedwith age, the imbalance determination parameter X without performing thesensor element section temperature elevating control even if the sensorelement section temperature elevating control should be performed (thatis, even if the value of the parameter obtaining permission flag Xkyokais “1”) (step 2410, step 2430, steps from step 1420 to step 1455, shownin FIG. 24).

Accordingly, since the fifth determination apparatus does not elevatethe air-fuel ratio sensor element temperature more than necessary, itcan perform accurate inter-cylinder air-fuel-ratio imbalancedetermination while avoiding the early deterioration of the air-fuelratio sensor.

It should be noted that the heater control means of the fifthdetermination apparatus (and the other apparatuses) may adopt/employ animpedance Zact of the solid electrolyte layer 671 as the valuecorresponding to the actual admittance Yact of the solid electrolytelayer 671, and control the amount of heat generation of the heater insuch a manner that a difference between the actual impedance Zact and atarget value (target impedance Ztgt) becomes smaller. In this case, theheater control means may be configured so as to realize the sensorelement section temperature elevating control by making the target value(the target impedance Ytgt) during the sensor element sectiontemperature elevating control is being performed different from (smallerthan) the target value during the sensor element section temperatureelevating control is not being performed.

Sixth Embodiment

Next, there will be described a determination apparatus according to asixth embodiment of the present invention (hereinafter simply referredto as the “sixth determination apparatus”).

The sixth determination apparatus obtains, as a usual temperatureair-fuel ratio fluctuation indicating amount Ztujo, the air-fuel ratiofluctuation indicating amount AFD while maintaining the air-fuel ratiosensor element temperature at the usual temperature (the firsttemperature); obtains, as an elevated temperature air-fuel ratiofluctuation indicating amount Ztup, the air-fuel ratio fluctuationindicating amount AFD while maintaining the air-fuel ratio sensorelement temperature at the elevated temperature (the secondtemperature); and performs the imbalance determination based on acomparison result between a value corresponding those values (e.g., adifference=Ztup Ztujo) and an imbalance determination threshold. Otherthan that, the sixth determination apparatus is the same as the firstdetermination apparatus.

(Actual Operation)

The CPU 71 of the sixth determination apparatus executes the routinesshown in FIGS. 12, 13, and 15, similarly to the CPU 71 of the firstdetermination apparatus. Further, the CPU 71 of the sixth determinationapparatus executes routines shown by flowcharts of “FIGS. 26 and 27” inplace of FIG. 14 every time a predetermined time (the sampling time ts)elapses. The routines shown in FIGS. 12, 13, and 15 have been alreadydescribed. Accordingly, the routines shown in FIGS. 26 and 27 will bedescribed hereinafter. It should be noted that each step in FIGS. 26 and27 at which the same processing is performed as each step which has beenalready described is given the same numeral as one given to such step.

It is assumed here that the value of the parameter obtaining permissionflag Xkyoka is set to (at) “1” owing to a first satisfaction of theparameter obtaining condition after the current start of the engine 10.In this case, the CPU 71 makes a “Yes” determination at step 1405 shownin FIG. 26 to proceed to step 2610, at which the CPU 71 determineswhether or not a value of a tentative flag Xkari is “0.” The value ofthe tentative flag Xkari is set to (at) “0” in the initial routinedescribed above.

Accordingly, the CPU 71 makes a “Yes” determination at step 2610 toproceed to step 2620, at which the CPU 71 sets the value of the sensorelement temperature elevation request flag Xtupreq to (at) “0.” As aresult, the air-fuel ratio sensor element temperature is maintained atthe usual temperature.

It should be noted that the value of the sensor element temperatureelevation request flag Xtupreq is set to (at) “0” in the initial routinedescribed above. Accordingly, the process of step 2620 at this stagedoes not change the value of the sensor element temperature elevationrequest flag Xtupreq substantially.

Thereafter, the CPU 71 obtains the air-fuel ratio fluctuation indicatingamount AFD by the processes of steps from step 1420 to step 1450. Thatis, the air-fuel ratio fluctuation indicating amount AFD is obtained inthe case in which the air-fuel ratio sensor element temperature is notelevated (the air-fuel ratio sensor element temperature is maintained atthe usual temperature).

After the air-fuel ratio fluctuation indicating amount AFD is obtainedat step 1450, the CPU 71 proceeds to step 2630 to determine whether ornot the value of the tentative flag Xkari is “0.” At the present pointin time, the value of the tentative flag Xkari is “0.” Accordingly, theCPU 71 makes a “Yes” determination at step 2630 to proceed to step 2640,at which the CPU 71 sets the value of the tentative flag Xkari to (at)“1.”

Subsequently, the CPU 71 proceeds to step 2650 to store the air-fuelratio fluctuation indicating amount AFD obtained at step 1450 as theusual temperature air-fuel ratio fluctuation indicating amount Ztujo(refer to FIG. 11). Thereafter, the CPU 71 proceeds to step 2695 to endthe present routine tentatively.

On the other hand, the CPU 71 starts processing from step 2700 shown inFIG. 27 at a predetermined timing, and determines at step 2710 whetheror not the present point in time is immediately after the value of theparameter obtainment completion flag Xobtain was changed from “0” to“1.” The value of the parameter obtainment completion flag Xobtain isset to (at) “0” in the initial routine described above. Further, at thispoint in time, the value of the parameter obtainment completion flagXobtain was not changed to “1.” Accordingly, the CPU 71 makes a “No”determination at step 2710 to directly proceed to step 2795 so as to endthe present routine tentatively.

Under this state, if the value of the parameter obtaining permissionflag Xkyoka is equal to “1”, the CPU 71 makes a “Yes” determination atstep 1405 shown in FIG. 26 when the CPU 71 proceeds to step 1405 toproceed to step 2610.

At this point in time, the value of the tentative flag Xkari is set to(at) “1.”, Accordingly, the CPU 71 makes a “No” determination at step2610 to proceed to step 2660, at which the CPU 71 sets the value of thesensor element temperature elevation request flag Xtupreq to (at)“1.”Consequently, the air-fuel ratio sensor element temperature iselevated to the elevated temperature by the execution of the routineshown in FIG. 15.

Subsequently, the CPU 71 proceeds to step 1415 to determine whether ornot the delay time Tdelayth has elapsed since a point in time at whichthe value of the sensor element temperature elevation request flagXtupreq was changed from “0” to “1.” When the delay time Tdelayth hasnot elapsed since the point in time at which the value of the sensorelement temperature elevation request flag Xtupreq was changed from “0”to “1”, the CPU 71 makes a “No” determination at step 1415 to directlyproceed to step 2695 so as to end the present routine tentatively.

On the other hand, if the delay time Tdelayth has elapsed since thepoint in time at which the value of the sensor element temperatureelevation request flag Xtupreq was changed from “0” to “1” when the CPU71 executes the process of step 1415 shown in FIG. 26, the CPU 71proceeds from step 1415 to steps following step 1420. Consequently, theair-fuel ratio fluctuation indicating amount AFD is obtained at step1450.

At this point in time, the value of the tentative flag Xkari is set to(at) “1.” Accordingly, when the CPU 71 proceeds to step 2630 followingstep 1450, the CPU 71 makes a “No” determination at step 2630 to proceedto step 2670, at which the CPU 71 sets the value of the parameterobtainment completion flag Xobtain to (at) “1.”

Subsequently, the CPU 71 proceeds to step 2680 to store/memorize theair-fuel ratio fluctuation indicating amount AFD obtained at step 1450,as the “elevated temperature air-fuel ratio fluctuation indicatingamount Ztup” (refer to FIG. 11). Thereafter, the CPU 71 proceeds to step2695 to end the present routine tentatively.

When the CPU 71 proceeds to step 2710 shown in FIG. 27 immediately afterthis point in time, the CPU 71 makes a “Yes” determination at step 2710to proceed to step 2720, since the present point in time is immediatelyafter the value of the parameter obtainment completion flag Xobtain waschanged from “0” to “1.”

The CPU 71 obtain, as the imbalance determination parameter DX, a “valueobtained by subtracting the usual temperature air-fuel ratio fluctuationindicating amount Ztujo from the elevated temperature air-fuel ratiofluctuation indicating amount Ztup” at step 2720. The imbalancedetermination parameter DX is a value which becomes larger as a degreeof difference between the elevated temperature air-fuel ratiofluctuation indicating amount Ztup and the usual temperature air-fuelratio fluctuation indicating amount Ztujo becomes larger. The imbalancedetermination parameter DX may be a ratio of the elevated temperatureair-fuel ratio fluctuation indicating amount Ztup to the usualtemperature air-fuel ratio fluctuation indicating amount Ztujo.Subsequently, the CPU 71 proceeds to step 2730 to determine whether ornot the imbalance determination parameter DX is larger than apredetermined imbalance determination threshold DXth.

When the imbalance determination parameter DX is larger than theimbalance determination threshold DXth, the CPU 71 makes a “Yes”determination at step 2730 to proceed to step 2740, at which the CPU 71sets the value of the imbalance occurrence flag XINB to “1.” That is,the CPU 71 determines that the inter-cylinder air-fuel-ratio imbalancestate has been occurring. Thereafter, the CPU 71 proceeds to step 2795to end the present routine tentatively.

In contrast, if the imbalance determination parameter DX is smaller thanor equal to the imbalance determination threshold DXth when the CPU 71executes the process of step 2730, the CPU 71 makes a “No” determinationat step 2730 to proceed to step 2750, at which the CPU 71 sets the valueof the imbalance occurrence flag XINB to “2.” That is, the CPU 71memorizes the “fact that it has been determined that the inter-cylinderair-fuel ratio imbalance state has not occurred according to the resultof the inter-cylinder air-fuel ratio imbalance determination.”Thereafter, the CPU 71 proceeds to step 2760 to set the value of thesensor element temperature elevation request flag Xtupreq to “0”, andproceeds to step 2795 to end the present routine tentatively. This stopsthe sensor element section temperature elevating control. It should benoted that step 2750 may be omitted.

As described above, the imbalance determining means of the sixthdetermination apparatus is configured so as to:

control the temperature of the sensor element section to the firsttemperature using the heater 678 during the parameter-obtaining-periodin which the predetermined parameter obtaining condition is satisfied(parameter obtaining permission flag Xkyoka=1) (refer to step 1405, step2610, and step 2620, shown in FIG. 26, step 1510 shown in FIG. 15, andthe “No” determination at step 1520 shown in FIG. 15), and obtain, asthe usual temperature air-fuel ratio fluctuation indicating amountZtujo, the value corresponding to the air-fuel ratio fluctuationindicating amount AFD which becomes larger as the fluctuation of theair-fuel ratio of said exhaust gas passing/flowing through the positionat which the air-fuel ratio sensor 67 is disposed becomes larger (stepsfrom step 1420 to step 1450, step 2630, and step 2650, shown in FIG.26);

control the temperature of the sensor element section to the “secondtemperature higher than the first temperature” using the heater 678during the parameter-obtaining-period (parameter obtaining permissionflag Xkyoka=1) (step 1405, step 2610, and step 2660 shown in FIG. 26,step 1510, step 1520, and step 1530, shown in FIG. 15), and obtain, asthe elevated temperature air-fuel ratio fluctuation indicating amountZtup, the value corresponding to an air-fuel ratio fluctuationindicating amount AFD which becomes larger as the fluctuation of theair-fuel ratio of said exhaust gas passing/flowing through the positionat which the air-fuel ratio sensor 67 is disposed becomes larger (stepsfrom step 1420 to step 1450, step 2630, and step 2680, shown in FIG.26);

further obtain, based on the elevated temperature air-fuel ratiofluctuation indicating amount Ztup and the usual temperature air-fuelratio fluctuation indicating amount Ztujo, the value which becomeslarger as the degree of the difference between the elevated temperatureair-fuel ratio fluctuation indicating amount Ztup and the usualtemperature air-fuel, ratio fluctuation indicating amount Ztujo becomeslarger, as the imbalance determination parameter DX (step 2720 shown inFIG. 27); and

determine that the inter-cylinder air-fuel-ratio imbalance state hasoccurred when the obtained imbalance determination parameter DX islarger than the predetermined imbalance determination threshold DXth,and determine that the inter-cylinder air-fuel-ratio imbalance state hasnot occurred when the obtained imbalance determination parameter DX issmaller than the predetermined imbalance determination threshold DXth(steps from step 2730 to step 2750, shown in FIG. 27).

As understood from FIG. 11, the value DX (e.g., DX=Ztup−Ztjujo), whichbecomes larger as the degree of the difference between the elevatedtemperature air-fuel ratio fluctuation indicating amount Ztup and theusual temperature air-fuel ratio fluctuation indicating amount Ztujobecomes larger, increases as the air-fuel ratio sensor elementtemperature becomes higher. Further, the value DX (=DX1) when theimbalance state is occurring (refer to the solid line L2) is larger thanthe value DX (=DX2) when the imbalance state is not occurring (refer tothe broken line L1). In addition, the difference between the value DX1and the value DX2 becomes larger as the difference between the elevatedtemperature (second temperature t2) and the usual temperature (firsttemperature t1) becomes larger.

Accordingly, as the sixth determination apparatus, when the values, eachcorresponding to the air-fuel ratio fluctuation indicating amounts, areobtained at the first temperature t1 as well as at the secondtemperature t2, and the imbalance determination is made based on thevalue which becomes larger as the degree of the difference between thosevalues, each corresponding those air-fuel ratio fluctuation indicatingamount (e.g., based on the difference DX between those values, or theratio Ztup/Ztujo, etc.), the imbalance determination can be performedaccurately.

It should be noted that the sixth determination apparatus obtainsfirstly the usual temperature air-fuel ratio fluctuation indicatingamount Ztujo, and thereafter, obtains the elevated temperature air-fuelratio fluctuation indicating amount Ztup, however, it may obtain firstlythe elevated temperature air-fuel ratio fluctuation indicating amountZtup, and thereafter, obtain the usual temperature air-fuel ratiofluctuation indicating amount Ztujo.

As described above, each of the determination apparatuses according toeach of the embodiments of the present invention can obtain theimbalance determination parameter which can accurately represent thedegree of the inter-cylinder air-fuel ratio imbalance state by elevatingthe temperature of the sensor element section of the air-fuel ratiosensor 67 when obtaining the imbalance determination parameter.Accordingly, each of the determination apparatuses according to each ofthe embodiments can accurately determine whether or not theinter-cylinder air-fuel ratio imbalance state has been occurring (hasoccurred).

The present invention is not limited to the above-described embodiments,and may be adopt various modifications within the scope of the presentinvention. For example, the air-fuel ratio fluctuation indicating amountAFD obtained as the imbalance determination parameter X, the elevatedtemperature air-fuel ratio fluctuation indicating amount Ztup, the usualtemperature air-fuel ratio fluctuation indicating amount Ztujo, and thelike” may be one of parameters described below.

(P1) The air-fuel ratio fluctuation indicating amount AFD may be a valuecorresponding to the trace/trajectory length of the output value Vabyfsof the air-fuel ratio sensor 67 (base indicating amount) or thetrace/trajectory length of the detected air-fuel ratio abyfs (baseindicating amount). For example, the trace length of the detectedair-fuel ratio abyfs may be obtained by obtaining the output valueVabyfs every elapse of the definite sampling time ts, converting theoutput value Vabyfs into the detected air-fuel ratio abyfs, andintegrating/accumulating an absolute value of a difference between thedetected air-fuel ratio abyfs and a detected air-fuel ratio abyfs whichwas obtained the definite sampling time ts before.

It is preferable that the trace length be obtained every elapse of theunit combustion cycle period. An average of the trace lengths for aplurality of the unit combustion cycle periods (i.e., the valuecorresponding to the trace length) may also be adopted as the air-fuelratio fluctuation indicating amount AFD. It should be noted that thetrace length of the output value Vabyfs or the trace length of thedetected air-fuel ratio abyfs has a tendency that they become larger asthe engine rotational speed becomes higher. Accordingly, when theimbalance determination parameter based on the trace length is used forthe imbalance determination, it is preferable that the imbalancedetermination threshold Xth be made larger as the engine rotationalspeed NE becomes higher.

(P2) The air-fuel ratio fluctuation indicating amount AFD may beobtained as a value corresponding to a base indicating amount which isobtained by obtaining a change rate of the change rate of the outputvalue Vabyfs of the air-fuel ratio sensor 67 or a change rate of thechange rate of the detected air-fuel ratio abyfs (i.e., a second-orderdifferential value of each of those values with respect to time). Forexample, the air-fuel ratio fluctuation indicating amount AFD may be amaximum value of absolute values of the “second-order differential value(d²(Vabyfs)/dt²) of the output value Vabyfs of the air-fuel ratio sensor67 with respect to time” in the unit combustion cycle period, or amaximum value of absolute values of the “second-order differential value(d²(abyfs)/dt²) of the detected air-fuel ratio abyfs represented by theoutput value Vabyfs of the upstream air-fuel ratio sensor 67 withrespect to time” in the unit combustion cycle period.

For example, the change rate of the change rate of the detected air-fuelratio abyfs may be obtained as follows.

-   -   The output value Vabyfs is obtained every elapse of the definite        sampling time ts.    -   The output value Vabyfs is converted into the detected air-fuel        ratio abyfs.    -   A difference between the detected air-fuel ratio abyfs and a        detected air-fuel ratio abyfs obtained the definite sampling        time ts before is obtained as the change rate of the detected        air-fuel ratio abyfs.    -   A difference between the change rate of the detected air-fuel        ratio abyfs and a change rate of the detected air-fuel ratio        abyfs obtained the definite sampling time is before is obtained        as the change rate of the change rate of the detected air-fuel        ratio abyfs (second-order differential value (d²((abyfs)/dt²).

In this case, among a plurality of the change rates of the change rateof the detected air-fuel ratio abyfs, that are obtained during the unitcombustion cycle period, a value whose absolute value is the largest maybe selected as a representing value. In addition, such a representingvalue may be obtained for each of a plurality of the unit combustioncycle periods. Further, an average of a plurality of the representingvalues may be adopted as the air-fuel ratio fluctuation indicatingamount AFD.

In addition, each of the determination apparatuses adopts thedifferential value d(abyfs)/dt (detected air-fuel ratio changing rateΔAF) as the base indicating amount, and adopts, as the air-fuel ratiofluctuation indicating amount AFD, the value based on the average of theabsolute values of the base indicating amounts in the unit combustioncycle period.

On the other hand, each of the determination apparatuses may obtain thedifferential value d(abyfs)/dt (detected air-fuel ratio changing rateΔAF) as the base indicating amount, obtain a value P1 whose absolutevalue is the largest among the differential values d(abyfs)/dt, each ofwhich is obtained in the unit combustion cycle period and has a positivevalue, obtain a value P2 whose absolute value is the largest among thedifferential values d(abyfs)/dt, each of which is obtained in the unitcombustion cycle period and has a negative value, and adopt a valuewhichever larger between the value P1 and the value P2, as the baseindicating amount. Then, the each of the determination apparatuses mayadopt, as the air-fuel ratio fluctuation indicating amount AFD, a meanvalue of absolute values of the base indicating amounts that areobtained in a plurality of unit combustion cycle periods.

Furthermore, each of the determination apparatuses described above maybe applied to a V-type engine. In such a case, the V-type engine maycomprise,

a right bank upstream catalyst disposed at a position downstream of anexhaust gas merging portion of two or more of cylinders belonging to aright bank (a catalyst disposed in the exhaust passage of the engine andat a position downstream of the exhaust gas merging portion into whichthe exhaust gases merge, the exhaust gases being discharged fromchambers of at least two or more of the cylinders among a plurality ofthe cylinders), and

a left bank upstream catalyst disposed at a position downstream of anexhaust gas merging portion of two or more of cylinders belonging to aleft bank (a catalyst disposed in the exhaust passage of the engine andat a position downstream of the exhaust merging portion into which theexhaust gases merge, the exhaust gases being discharged from chambers oftwo or more of the cylinders among the rest of the at least two or moreof the cylinders).

Further, the V-type engine may comprise an upstream air-fuel ratiosensor for the right bank and a downstream air-fuel ratio sensor for theright bank disposed upstream and downstream of the right bank upstreamcatalyst, respectively, and may comprise upstream air-fuel ratio sensorfor the left bank and a downstream air-fuel ratio sensor for the leftbank disposed upstream and downstream of the left bank upstreamcatalyst, respectively. Each of the upstream air-fuel ratio sensors,similarly to the air-fuel ratio sensor 67, is disposed between theexhaust gas merging portion of each of the banks and the upstreamcatalyst of each of the banks. In this case, a main feedback control forthe right bank and a sub feedback for the right bank are performed basedon the output values of the upstream air-fuel ratio sensor for the rightbank and the downstream air-fuel ratio sensor for the right bank, and amain feedback control for the left bank and a sub feedback for the leftbank are independently performed based on the output values of theupstream air-fuel ratio sensor for the left bank and the downstreamair-fuel ratio sensor for the left bank.

Further, in this case, the determination apparatus may obtain “anair-fuel ratio fluctuation indicating amount AFD (an imbalancedetermination parameter X)” for the right bank based on the output valueof the upstream air-fuel ratio sensor for the right bank, and maydetermine whether or not an inter-cylinder air-fuel ratio imbalancestate has been occurring among the cylinders belonging to the right bankusing those values.

Similarly, the determination apparatus may obtain “an air-fuel ratiofluctuation indicating amount AFD (an imbalance determination parameterX)” for the left bank based on the output value of the upstream air-fuelratio sensor for the left bank, and may determine whether or not aninter-cylinder air-fuel ratio imbalance state has been occurring amongthe cylinders belonging to the left bank using those values.

In addition, each of the determination apparatuses may change theimbalance determination threshold Xth (including the high-side thresholdXHith and the low-side threshold XLoth) in such a manner that thethreshold Xth becomes larger as the intake air-flow rate Ga becomeslarger. This is because the responsiveness of the air-fuel ratio sensor67 becomes lower as the intake air-flow rate Ga becomes smaller due tothe presence of the protective covers 67 b and 67 c.

Furthermore, it is preferable that the high-side threshold XHith beequal to or larger than the imbalance determination threshold Xth, andthe low-side threshold XLoth be equal to or smaller than the imbalancedetermination threshold Xth. It should be noted that the high-sidethreshold XHith may be smaller than the imbalance determinationthreshold Xth, if it can be clearly determined that the inter-cylinderair-fuel ratio imbalance state has been occurring when the tentativeparameter Xz is larger than the high-side threshold XHith. Similarly,the low-side threshold XLoth may be a value which allows the apparatusto clearly determine that the inter-cylinder air-fuel ratio imbalancestate has not been occurring when the tentative parameter Xz is smallerthan the low-side threshold XLoth.

Further, each of the determination apparatuses comprises indicated fuelinjection amount control means for controlling the indicated fuelinjection amount in such a manner that the air-fuel ratio of the mixturesupplied to the combustion chambers of the two or more of the cylinderscoincides with the target air-fuel ratio (routines shown in FIGS. 12 and13). The instructed fuel injection amount control means includesair-fuel ratio feedback control means for calculating the air-fuel ratiofeedback amount (DFi), based on the air-fuel ratio (detected air-fuelratio abyfs) represented by the output value Vabyfs of the air-fuelratio sensor 67 and the target air-fuel ratio abyfr, in such a mannerthat those values become equal to each other, and for determining(adjusting, controlling) the instructed fuel injection amount based onthe air-fuel ratio feedback amount (DFi) (step 1240 shown in FIG. 12 andthe routine shown in FIG. 13). In addition, the instructed fuelinjection amount control means may be feedforward control means, forexample, for determining (controlling), as the instructed fuel injectionamount, a value obtained by dividing the in-cylinder intake air amount(air amount taken into a single cylinder per one intake stroke) Mcdetermined based on the intake air flow rate and the engine rotationalspeed by the target air-fuel ratio abyfr without including the air-fuelratio feedback control means. That is, the air-fuel ratio feedbackamount DFi shown in FIG. 12 may be set to (at) “0.”

Furthermore, the heater control means of each of the determinationapparatuses described above may be configured so as to set the heaterduty Duty to 100% (i.e., to set the amount of energy supplied to theheater 678 to the maximum value) when the actual admittance Yact issmaller than the “value obtained by subtracting the predeterminedpositive value a from the target admittance Ytgt”, set the heater dutyDuty to “0” (i.e., to set the amount of energy supplied to the heater678 to the minimum value) when the actual admittance Yact is larger thanthe “value obtained by adding the predetermined positive value a to thetarget admittance Ytgt”, and set the heater duty Duty to a“predetermined value (e.g., 50%) larger than 0 and smaller than 100%”when the actual admittance Yact is between the “value obtained bysubtracting the predetermined positive value a from the targetadmittance Ytgt” and the “value obtained by adding the predeterminedpositive value a to the target admittance Ytgt.”

1. An inter-cylinder air-fuel ratio imbalance determination apparatusfor an internal combustion engine, applied to a multi-cylinder internalcombustion engine having a plurality of cylinders, comprising: anair-fuel ratio sensor, which is disposed at a position in an exhaustmerging portion of an exhaust passage of said engine into which exhaustgases discharged from at least two or more cylinders among a pluralityof said cylinders merge or disposed in said exhaust passage at aposition downstream of said exhaust merging portion, and which includesa solid electrolyte layer, an exhaust-gas-side electrode layer which isformed on one of surfaces of said solid electrolyte layer, a diffusionresistance layer which covers said exhaust-gas-side electrode layer andwhich said exhaust gases reach, an atmosphere-side electrode layer whichis formed on the other one of said surfaces of said solid electrolytelayer, and a heater which heats a sensor element section including saidsolid electrolyte layer, said exhaust-gas-side electrode layer, and saidatmosphere-side electrode layer, wherein, when a predetermined voltageis applied between said exhaust-gas-side electrode layer and saidatmosphere-side electrode layer, said air-fuel ratio sensor outputs,based on a limiting current flowing through said solid electrolytelayer, an output value corresponding to an air-fuel ratio of an exhaustgas passing through said position at which said air-fuel ratio sensor isdisposed; a plurality of fuel injection valves, each of which isdisposed in such a manner that it corresponds to each of said at leasttwo or more of said cylinders, and each of which injects fuel, containedin an air-fuel mixture supplied to each of combustion chambers of saidtwo or more of said cylinders, in an amount in accordance with aninstructed fuel injection amount; heater control unit which isconfigured to control an amount of heat generation by said heater;imbalance determining unit which is configured to obtain, based on saidoutput value of said air-fuel ratio sensor, an imbalance determinationparameter which becomes larger as a fluctuation of an air-fuel ratio ofsaid exhaust gas passing through said position at which said air-fuelratio sensor is disposed becomes larger, in a parameter-obtaining-periodwhich is a period in which a predetermined parameter obtaining conditionis being satisfied, to determine that an inter-cylinder air-fuel ratioimbalance state has occurred when said obtained imbalance determinationparameter is larger than a predetermined imbalance determinationthreshold, and to determine that said inter-cylinder air-fuel ratioimbalance state has not occurred when said obtained imbalancedetermination parameter is smaller than said imbalance determinationthreshold; wherein, said imbalance determining unit is configured tomake the heater control unit perform a sensor element sectiontemperature elevating control to have a temperature of said air-fuelratio sensor element for said parameter-obtaining-period be higher thana temperature of said air-fuel ratio sensor element for a period otherthan said parameter-obtaining-period.
 2. The inter-cylinder air-fuelratio imbalance determination apparatus according to claim 1, whereinsaid imbalance determining unit is configured to: obtain, based on saidoutput value of said air-fuel ratio sensor, said imbalance determinationparameter as a tentative parameter before having said heater controlunit perform said sensor element section temperature elevating controlin said parameter-obtaining-period; determine that said inter-cylinderair-fuel ratio imbalance state has been occurred when said obtainedtentative parameter is larger than a predetermined high-side threshold;determine that said inter-cylinder air-fuel ratio imbalance state hasnot occurred when said obtained tentative parameter is smaller than alow-side threshold which is smaller by a predetermined value than saidhigh-side threshold; withhold a determination as to whether or not saidinter-cylinder air-fuel-ratio imbalance state has occurred when saidobtained tentative parameter is between said high-side threshold andsaid low-side threshold; have said heater control unit perform saidsensor element section temperature elevating control during saidparameter-obtaining-period, and obtain, based on said output value ofsaid air-fuel ratio sensor, said imbalance determination parameter as afinal parameter, while said determination as to whether or not saidinter-cylinder air-fuel-ratio imbalance state has occurred is beingwithheld; and determine that said inter-cylinder air-fuel-ratioimbalance state has occurred when said obtained final parameter islarger than said imbalance determination threshold, and determine thatsaid inter-cylinder air-fuel-ratio imbalance state has not occurred whensaid obtained final parameter is smaller than said imbalancedetermination threshold.
 3. The inter-cylinder air-fuel ratio imbalancedetermination apparatus according to claim 1, wherein said imbalancedetermining unit is configured to start to obtain said imbalancedetermination parameter after a predetermined delay time has elapsedsince a point in time at which said sensor element section temperatureelevating control was started.
 4. The inter-cylinder air-fuel ratioimbalance determination apparatus according to claim 3, wherein saidimbalance determining unit is configured to set said predetermined delaytime in such a manner that said delay time is shorter as a temperatureof said exhaust gas is higher.
 5. The inter-cylinder air-fuel ratioimbalance determination apparatus according to claim 3, wherein saidimbalance determining unit is configured to set said predetermined delaytime in such a manner that said delay time is shorter as an intake airflow rate of said engine or a load of said engine is greater.
 6. Theinter-cylinder air-fuel ratio imbalance determination apparatusaccording to claim 1, wherein said imbalance determining unit isconfigured so as to have said heater control unit start to perform saidsensor element section temperature elevating control at a point in timeat which a warming-up of said engine is completed after a start of saidengine, and have said heater control unit finish said sensor elementsection temperature elevating control at a point in time at whichobtaining said imbalance determination parameter is completed.
 7. Theinter-cylinder air-fuel ratio imbalance determination apparatusaccording to claim 1, wherein said heater control unit is configured tocontrol said amount of heat generation of said heater in such a mannerthat a difference between a value corresponding to an actual admittanceof said solid electrolyte layer and a target value is decreased, and torealize said sensor element section temperature elevating control bymaking said target value while said sensor element section temperatureelevating control is being performed different from said target valuewhile said sensor element section temperature elevating control is notbeing performed; and said imbalance determining unit is configured todetermine whether or not said air-fuel ratio has deteriorated with age,and obtain, when it is determined that said air-fuel ratio hasdeteriorated with age, said imbalance determination parameter withoutperforming said sensor element section temperature elevating controleven when said sensor element section temperature elevating controlshould be performed.
 8. An inter-cylinder air-fuel ratio imbalancedetermination apparatus for an internal combustion engine, applied to amulti-cylinder internal combustion engine having a plurality ofcylinders, comprising: an air-fuel ratio sensor, which is disposed at aposition in an exhaust merging portion of an exhaust passage of saidengine into which exhaust gases discharged from at least two or morecylinders among a plurality of said cylinders merge or disposed in saidexhaust passage at a position downstream of said exhaust mergingportion, and which includes a solid electrolyte layer, anexhaust-gas-side electrode layer which is formed on one of surfaces ofsaid solid electrolyte layer, a diffusion resistance layer which coverssaid exhaust-gas-side electrode layer and which said exhaust gasesreach, an atmosphere-side electrode layer which is formed on the otherone of said surfaces of said solid electrolyte layer, and a heater whichheats a sensor element section including said solid electrolyte layer,said exhaust-gas-side electrode layer, and said atmosphere-sideelectrode layer, wherein, when a predetermined voltage is appliedbetween said exhaust-gas-side electrode layer and said atmosphere-sideelectrode layer, said air-fuel ratio sensor outputs, based on a limitingcurrent flowing through said solid electrolyte layer, an output valuecorresponding to an air-fuel ratio of an exhaust gas passing throughsaid position at which said air-fuel ratio sensor is disposed; aplurality of fuel injection valves, each of which is disposed in such amanner that it corresponds to each of said at least two or more of saidcylinders, and each of which injects fuel, contained in an air-fuelmixture supplied to each of combustion chambers of said two or more ofsaid cylinders, in an amount in accordance with an instructed fuelinjection amount; imbalance determining unit which is configured to:control a temperature of said sensor element section to a firsttemperature using said heater during a parameter-obtaining-period inwhich a predetermined parameter obtaining condition is satisfied, andobtain, as a usual temperature air-fuel ratio fluctuation indicatingamount, a value corresponding to an air-fuel ratio fluctuationindicating amount which becomes larger as a fluctuation of an air-fuelratio of said exhaust gas passing through said position at which saidair-fuel ratio sensor is disposed becomes larger; control saidtemperature of said sensor element section to a second temperaturehigher than said first temperature using said heater during saidparameter-obtaining-period, and obtain, as an elevated temperatureair-fuel ratio fluctuation indicating amount, said value correspondingto said air-fuel ratio fluctuation indicating amount which becomeslarger as said fluctuation of said air-fuel ratio of said exhaust gaspassing through said position at which said air-fuel ratio sensor isdisposed becomes larger; and obtain, based on said elevated temperatureair-fuel ratio fluctuation indicating amount and said usual temperatureair-fuel ratio fluctuation indicating amount, a value which becomeslarger as a degree becomes larger of a difference between said elevatedtemperature air-fuel ratio fluctuation indicating amount and said usualtemperature air-fuel ratio fluctuation indicating amount, as animbalance determination parameter, and determine that an inter-cylinderair-fuel-ratio imbalance state has occurred when said obtained imbalancedetermination parameter is larger than a predetermined imbalancedetermination threshold, and determine that said inter-cylinderair-fuel-ratio imbalance state has not occurred when said obtainedimbalance determination parameter is smaller than said imbalancedetermination threshold.
 9. The inter-cylinder air-fuel ratio imbalancedetermination apparatus according to claim 1, wherein, said air-fuelratio detecting section of said air-fuel ratio sensor includes acatalytic section which accelerates an oxidation-reduction reaction andhas an oxygen storage function, and said air-fuel ratio sensor isconfigured to have said exhaust gas passing through said exhaust passagereach said diffusion resistance layer through said catalytic section.10. The inter-cylinder air-fuel ratio imbalance determination apparatusaccording to claim 1, wherein, said air-fuel ratio sensor furthercomprises a protective cover, which accommodates said air-fuel ratiodetecting section to cover said air-fuel ratio detecting section in itsinside, and which includes an inflow hole for allowing said exhaust gasflowing through said exhaust passage to flow into said inside and anoutflow hole for allowing said exhaust gas which has flowed into saidinside to flow out to said exhaust passage.
 11. The inter-cylinderair-fuel ratio imbalance determination apparatus according to claim 10,wherein, said imbalance determining unit is configured to obtain, as abase indicating amount, a time differential value of said output valueof said air-fuel ratio sensor or of a detected air-fuel ratiorepresented by said output value, and obtain said imbalancedetermination parameter based on said obtained base indicating amount.12. The inter-cylinder air-fuel ratio imbalance determination apparatusaccording to claim 10, wherein, said imbalance determining unit isconfigured to obtain, as a base indicating amount, a time second-orderdifferential value of said output value of said air-fuel ratio sensor orof a detected air-fuel ratio represented by said output value, andobtain said imbalance determination parameter based on said obtainedbase indicating amount.